Method and apparatus for initialization of a wellbore survey tool via a remote reference source

ABSTRACT

A method and an apparatus are provided for determining an orientation of a wellbore survey tool at a first position with respect to a reference direction. At least one first signal is indicative of an orientation of a directional reference system with respect to the reference direction. The directional reference system is positioned at a second position spaced from the first position. At least one second signal is indicative of a relative orientation of the wellbore survey tool with respect to the directional reference system. The orientation of the wellbore survey tool at the first position is determined in response at least in part to the at least one first signal and the at least one second signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/555,737, filed Sep. 8, 2009 and incorporated in its entiretyby reference herein, which claims the benefit of priority from U.S.Provisional Appl. Nos. 61/180,779 filed May 22, 2009 and 61/186,748filed Jun. 12, 2009, both of which are incorporated in their entirety byreference herein. This application also claims the benefit of priorityfrom U.S. Provisional Appl. No. 61/450,073 filed Mar. 7, 2011, which isincorporated in its entirety by reference herein.

BACKGROUND

1. Field

The present application relates generally to methods and apparatus forinitialization of a wellbore survey tool.

2. Description of the Related Art

There are typically two types of surveying by which wellbore surveytools conduct surveys (e.g., gyroscopic- or gyro-based surveys) ofwellbores. The first type is static surveying, in which measurements ofthe Earth's rotation are taken at discrete depth intervals along thewell trajectory. These measurements can be used to determine theorientation of the survey tool with respect to a reference vector, suchas the vector defined by the horizontal component of the Earth's rate inthe direction of the axis of the Earth's rotation; a process alsoreferred to herein as gyro-compassing. The second type is continuoussurveying, in which the gyroscopic or gyro measurements are used todetermine the change in orientation of the survey tool as it traversesthe well trajectory. This process uses the gyro measurements of turnrate with respect to a known start position. The start position may bederived, for example, by conducting a static survey prior to enteringthe continuous survey mode (which may also be referred to as anautonomous or autonomous/continuous survey mode).

Under certain circumstances, static surveying generally becomes lessaccurate than in other circumstances. For example, when operating athigh latitudes on the Earth's surface the static survey process becomesless accurate than at low latitudes. At relatively high latitudes, thereference vector to which the survey tool aligns itself during thegyro-compassing procedure, the horizontal component of Earth's rate(Ω_(H)), is small compared to the value in equatorial and mid-latituderegions, as indicated by the following equation:

Ω_(H)=Ω cos L,  (Eq. 1)

where Ω=Earth's rate and L=latitude. Generally, a satisfactorydirectional survey can be achieved using gyro-compassing at latitudes ofup to about 60 degrees. However, the accuracy can degrade rapidlythereafter as the cosine of latitude reduces more rapidly and themagnitude of Ω_(H) thus becomes much smaller. FIG. 1 schematicallyillustrates the horizontal component Ω_(H) of the Earth's rate forchanging latitude. As shown, at zero latitude Ω_(H) is at its maximumvalue and is equal to the Earth's rate (a). Ω_(H) successively decreasesto Ω_(H)=Ω cos L₁ and Ω_(H)=Ω cos L₂ for increasing latitudes L₁ and L₂,respectively, and Ω_(H) is zero at 90 degrees of latitude (i.e., at theNorth Pole). There is a significant amount of oil and gas exploration atrelatively high latitudes (e.g., latitudes in excess of 70 degrees). Atthese latitudes, the accuracy of well surveys based on gyro-compassingcan be degraded. Similar degradations in survey accuracy can also occurwhen using magnetic survey tools instead of, or in addition to,gyro-based survey tools. As such, survey accuracy may similarly decreaseat locations close to the Earth's magnetic poles when using magneticsurvey tools.

In addition, the accuracy of gyro-compassing can be degraded whenconducted from a moving platform (e.g., an offshore platform), ascompared to being conducted from a relatively static platform. Forexample, during operation from a moving platform, the survey tool willbe subjected to platform rotational motion in addition to the Earth'srotation. Under such conditions, tool orientation with respect to thehorizontal Earth's rate vector (Ω_(H)) may be difficult to determinewith the precision that is possible on a stationary platform since thedirectional reference, defined by Ω_(H), is effectively corrupted by theplatform motion.

SUMMARY

In certain embodiments, a method is provided for determining anorientation of a wellbore survey tool at a first position with respectto a reference direction. The method comprises receiving at least onefirst signal indicative of an orientation of a directional referencesystem with respect to the reference direction. The directionalreference system is positioned at a second position spaced from thefirst position. The method further comprises receiving at least onesecond signal indicative of a relative orientation of the wellboresurvey tool with respect to the directional reference system. The methodfurther comprises determining the orientation of the wellbore surveytool at the first position in response at least in part to the at leastone first signal and the at least one second signal.

In certain embodiments, a system for determining an orientation of awellbore survey tool is provided. The system comprises one or morecomputer processors. The system further comprise one or more inputsconfigured to receive data indicative of an orientation of a directionalreference system with respect to a reference direction and dataindicative of a relative orientation of the wellbore survey tool withrespect to the directional reference system. The direction referencesystem is positioned at a first position relative to a wellbore entranceand a wellbore survey tool is mounted at a second position relative tothe wellbore entrance spaced away from the first position. The systemfurther comprises a wellbore initialization module executing in the oneor more computer processors and configured to, in response at least inpart to the received data, calculate an orientation of the survey tool.

In certain embodiments, a system for use in determining an orientationof a wellbore survey tool is provided. The system comprises at least onedirectional reference system configured to provide data indicative of anorientation of the at least one directional reference system withrespect to a reference direction. The system further comprise an opticalcomponent mounted at a predetermined orientation with respect to thedirectional reference system and configured to transmit light along aline extending between the directional reference system and a firstreflecting surface mounted at a predetermined orientation with respectto the wellbore survey tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the horizontal component of the Earth'srate for changing latitude.

FIG. 2 schematically illustrates an example apparatus for initializing awellbore survey tool in accordance with certain embodiments describedherein.

FIG. 3 schematically illustrates apparatus according to certainembodiments described herein in a first location in which a relativelyclear communication path between GPS antennae of the apparatus and GPSsatellites, and in a second location in which the GPS antennae are atleast partially shielded from communication with GPS satellites by aderrick.

FIG. 4 schematically illustrates another example apparatus in accordancewith certain embodiments described herein.

FIG. 5 schematically illustrates a top view of an apparatus including anintegrated GPS/AHRS unit in accordance with certain embodimentsdescribed herein.

FIGS. 6A-6C schematically illustrate top, front and right side views,respectively, of an apparatus including a tool positioning element inaccordance with certain embodiments herein.

FIG. 6D schematically illustrates a partial perspective view of anapparatus including a tool positioning element during positioning of asurvey tool in accordance with certain embodiments described herein.

FIG. 7 schematically illustrates an example wellbore survey tool onwhich a directional reference system is directly mounted in accordancewith certain embodiments described herein.

FIG. 8 is a flow diagram illustrating an example wellbore survey toolinitialization process in accordance with certain embodiments describedherein.

FIG. 9 is a flowchart of an example method of initializing a wellboresurvey tool in accordance with certain embodiments described herein.

FIG. 10 is a flowchart of an example method of initializing a wellboresurvey tool utilizing an angular rate matching procedure in accordancewith certain embodiments described herein.

FIG. 11 schematically illustrates an example apparatus for moving awellbore survey tool in accordance with certain embodiments describedherein.

FIG. 12 is a flowchart of an example method for determining anorientation of a wellbore survey tool at a first position with respectto a reference direction in accordance with certain embodimentsdescribed herein.

FIG. 13 illustrates an example survey tool initialization configurationincluding a survey tool and a reference system and also illustrates acorresponding initialization process, according to certain embodimentsdescribed herein.

FIG. 14 illustrates an example survey tool mounted vertically and havinga mirror attached to the tool, according to certain embodimentsdescribed herein.

FIG. 15 illustrates an example survey tool mounted horizontally in av-block mount, according to certain embodiments described herein.

FIG. 16 illustrates an example survey tool initialization configurationin which a reference system is mounted on a platform along with one ormore optical sighting instruments, according to certain embodimentsdescribed herein.

FIGS. 17A and 17B illustrate example initialization configurations inwhich a reference system is mounted on a platform along with one or moreoptical sighting instruments and a survey tool, according to certainembodiments described herein.

FIG. 18 illustrates an example initialization configuration in which anautocollimation device is mounted at a predetermined orientation withrespect to a reference system and is used to determine the initialorientation of the survey tool.

FIG. 19 illustrates an example survey tool initialization configurationin which a sleeve is affixed to a survey tool, according to certainembodiments described herein.

FIG. 20 illustrates another example survey tool initializationconfiguration in which a sleeve is affixed to a survey tool and thetool/sleeve assembly are keyed into a clamping mechanism, according tocertain embodiments described herein.

FIG. 21 shows an example rig having a survey tool and reference systemmounted thereon, according to certain embodiments described herein.

FIGS. 22 and 23 shows further example initialization configurationsincluding inertial attitude and heading reference systems (AHRS),according to certain embodiments described herein.

DETAILED DESCRIPTION

Embodiments described herein provide systems and methods which generallyallow precision well surveys to be conducted at high latitude locations,from a moving surface (e.g., an off-shore moving platform), or both.

A. Overview

While underground, gyro survey tools generally rely upon gyro-compassingto conduct a static survey and/or to initiate a period of continuoussurveying to determine the orientation of the survey tool with respectto a reference vector (e.g., the vector defined by the horizontalcomponent of the Earth's rate). However, at the surface, there are otherprocedures which may be adopted. For example, land surveying techniquescan be used to define a reference direction (which may also be referredto as a “benchmark direction”) to which the tool can be aligned. Thisprocess may be referred to as fore-sighting.

Alternatively, measurements from a directional reference system, such asa satellite navigation system, may be used to determine the orientation(e.g., the attitude) of a survey tool with respect to a known geographicreference frame. The Global Positioning System (GPS) or the equivalentsystem developed by the former Soviet Union, the Global NavigationSatellite System (GLONASS), may be used, for example. Systems existwhich use measurements of the differences in carrier wave phase betweentwo or more receiving antennae spaced a known distance apart todetermine the attitude of the body or vehicle on which the antennae aremounted. Examples of such systems are described, for example, in U.S.Pat. No. 5,534,875, entitled “Attitude Determining System for Use withGlobal Positioning System”, which is incorporated in its entirety byreference herein. These systems provide world-wide measurement ofposition, velocity and attitude on and above the surface of the Earthand are substantially immune to magnetic deviations and anomalies.

Using such systems in accordance with certain embodiments describedherein, the initial orientation (e.g., attitude) of a survey tool maythus be defined accurately while above ground (e.g., on the surface) anddata indicative of the initial orientation (e.g., attitude data) canthen be transferred to the tool. In certain circumstances, the surveytool may then be switched to continuous survey mode prior to beingpositioned for insertion into the wellbore and/or prior to insertioninto the wellbore. For example, the initial orientation of the tool maybe measured prior to pick-up of the survey tool (e.g., from horizontalto vertical with respect to the wellbore) to position the survey toolinto the wellbore. In certain embodiments, this initial measurement maybe made while the tool is positioned generally horizontally with respectto the wellbore (e.g., laying on a surface in the vicinity of thewellbore), for example. The survey tool may be switched to continuousmode such that its subsequent orientation (e.g., heading, trajectory,attitude, azimuth, etc.) can be measured with respect to the initialorientation. The survey tool may then be lifted from the horizontalposition to another position, such as a vertical position. A continuoussurvey of the wellbore may then be conducted as the survey tooltraverses the well trajectory.

Both land surveying techniques and methods using satellite navigationtechniques for determining an initial orientation of the survey tool aresusceptible to human errors under certain conditions. For example, thetool may be picked up relatively rapidly and one or more of the sensorskeeping track of the orientation of the tool (e.g., in continuous surveymode) may become saturated or otherwise reach their rate limits. Inaddition, the tool may be dropped in some cases. Certain embodimentsdescribed herein address such problems by linking a survey/GPS referencewith an inertial system in the survey tool through a semi-automated orautomated process that can operate both at high latitude and on a movingsurface (e.g., a moving off-shore drilling rig). For example, someembodiments enable the movement of a wellbore tool in a controlledmanner (e.g., at a controlled rate) with respect to the wellbore (e.g.,through an automated or semi-automated process) and while the tool is incontinuous mode after determining an initial orientation (e.g., using aGPS system).

In general, a wellbore survey tool (e.g., a gyro survey tool) may beoperated under at least the following categories of conditions:

-   -   (1) Operation from a fixed, non-moving platform at limited        borehole inclination. In such conditions, for example, one        approach is to use a two axis (xy) gyro system to conduct static        gyro-compassing surveys. In addition, continuous surveys may be        initiated (e.g., using gyro-compassing) and conducted over the        whole, or sections, of the wellbore.    -   (2) Operation in high inclination boreholes from a fixed        platform. Under these conditions, for example, one approach is        to use a three axis (xyz) gyro system to conduct static        gyro-compassing surveys. In addition, continuous surveys may be        initiated (e.g., using gyro-compassing) and conducted over the        whole, or sections, of the wellbore.    -   (3) Operation at high latitude from a fixed platform. Here,        continuous surveys may be used as the survey tool passes along        the wellbore. The survey may be initiated (e.g., an initial        orientation may be determined), at the surface using techniques        described herein (e.g., using satellite navigation such as GPS)        in accordance with embodiments herein. In certain embodiments,        satellite navigation techniques may be used in conjunction with        an inertial navigation system (INS) (e.g., a joint GPS/INS        system, or a stand alone inertial navigation system) which can        address issues such as satellite signal non-availability or        shielding described herein.    -   (4) Operation on or from a moving surface (e.g., on or from an        off-shore drilling rig). In such conditions, and in accordance        with embodiments described herein, continuous surveys may be        used throughout the wellbore. The survey may be initiated (e.g.,        an initial orientation may be determined) at the surface using        satellite navigation. In certain embodiments, satellite        navigation techniques may be used in conjunction with an        inertial navigation system (INS) (e.g., a joint GPS/INS system,        or a stand alone inertial navigation system) which can address        issues such as satellite signal non-availability or shielding as        described herein, and to aid transfer of satellite reference        data to the survey tool. Angular matching techniques described        herein may also be used to improve the accuracy of the survey.

In certain embodiments, an apparatus (e.g., a rigid platform structure)is configured to be attached to a wellbore surveying tool and to bemoved between multiple positions on a drilling rig. The apparatus can beconfigured to allow for accurate initialization of the surveymeasurement system within the wellbore survey tool. The apparatus may beconfigured to enable the transfer of relatively precise orientation(e.g., attitude and/or azimuth) data to a directional survey system inthe wellbore survey tool for drilling operations, such as drillingoperations at high latitude locations on the Earth, or when operatingoff-shore from a moving drilling rig.

Certain embodiments described herein provide a relatively precisedetermination of the orientation of a wellbore survey tool (e.g.,attitude, azimuth and/or heading reference) at the surface which doesnot use gyro-compassing. In certain embodiments, this orientationinformation may be transferred to an inertial system in the survey tool.This technique can be performed by devices that generally operateindependently of the instrumentation and equipment within the surveytool. This independent orientation determination may be performed, forexample, based on established land surveying methods (e.g.,fore-sighting) or the use of satellite based information (e.g., usingGPS technology), and/or using inertial navigation systems (e.g., usingan attitude and heading reference system (AHRS) unit). Once theorientation (e.g., attitude and/or azimuth) data is transmitted to thesurvey tool, a continuous survey procedure can be initiated whichinvolves the integration of gyro measurements as the survey tool isplaced in a bore-hole and as it traverses the well path. This continuoussurveying process is generally initiated or initialized by theorientation data (e.g., attitude, azimuth, and/or heading data) derivedat the surface.

To enable these functions while avoiding potential problems that canoccur when surveying underground bore-holes, apparatus (e.g., platformstructures) as described herein can be moved to a drilling rig generallyanywhere in the world where it can be set up to accommodate the variousitems of equipment used to perform the orientation determination (e.g.,attitude, azimuth and/or heading reference determination). Theseapparatus may comprise rigid platform structures, be of relatively lowweight, and may be capable of being mounted generally rigidly on thedrilling rig at a location(s) alongside or close to the well head.

The apparatus described herein can include fixturing (e.g., one or moremounts) to allow both independent surface reference equipment (e.g., adirectional reference system such as a GPS receiver with two or moreantennae) and the survey tool to be mounted (e.g., relatively rigidly)on or within the apparatus. In certain embodiments, the apparatus can beleveled and the orientation of the survey tool can be aligned relativelyprecisely to a reference direction defined on the platform by thesurface reference equipment (e.g., defined by the relative positioningof two or more antennae in the case of a GPS reference). In oneembodiment, a GPS receiver is capable of determining the direction ofthe line joining two antennae of the GPS receiver with respect to truenorth. In this situation, the azimuth angle defined by the GPS (e.g.,the angle of the line joining the two antennae with respect to truenorth) can be transferred to the survey tool. Inclination and tool-faceangle of the survey tool can additionally be determined based onmeasurements provided by the survey tool (e.g., by one or moreaccelerometers within the survey tool). The initial orientation (e.g.,azimuth, inclination and tool-face angles) can be thereby determined andused to initialize the subsequent integration process (e.g., duringcontinuous surveying) that can be implemented within the tool forkeeping track of bore-hole direction as the tool moves along itstrajectory. In general, the orientation information can be madeavailable independent or regardless of the latitude of the drillingplatform.

B. Initialization of the Survey Tool at High Latitudes

FIG. 2 schematically illustrates an example apparatus 10 forinitializing a wellbore survey tool 30 in accordance with certainembodiments described herein. In certain embodiments, the apparatus 10comprises a base portion 12 and a first mounting portion 14 mechanicallycoupled to the base portion 12. The first mounting portion 14 of certainembodiments is adapted to be mechanically coupled to at least onedirectional reference system 16. The at least one directional referencesystem 16 can be configured to provide data indicative of an orientation(e.g., attitude and/or azimuth) of the at least one directionalreference system 16 with respect to a reference direction 18. Thereference direction 18 may be north (e.g., true or rotational north ormagnetic north). In certain embodiments, the apparatus 10 furthercomprises a second mounting portion 20 mechanically coupled to the baseportion 12. The second mounting portion 20 may be configured to bemechanically coupled to the wellbore survey tool 30 such that thewellbore survey tool 30 has a predetermined orientation with respect tothe at least one directional reference system 16. For example, as shownin FIG. 2, the survey tool 30 may be substantially parallel to thedirectional reference system 16. In other embodiments, the survey tool30 may be oriented at some predetermined angle relative to thedirectional reference system 16, or may be oriented in some otherpredetermined fashion with respect to the directional reference system16.

As shown in FIG. 2, the base portion 12 may comprise a substantiallyrigid, generally rectangular platform structure including a generallyplanar surface 13. In other embodiments, the base portion 12 may have adifferent shape (e.g., circular, ovular, trapezoidal, etc.), may besomewhat flexible, and/or may include one or more inclined surfaces,declined surfaces, stepped portions, etc.

In certain embodiments, the base portion 12 comprises carbon fiber. Inother configurations, the base portion 12 may comprise another materialsuch as steel, other metal, or a polymer or plastic material. In certainembodiments, the first mounting portion 14 comprises an area of the baseportion 12 on which the directional reference system 16 can be mounted.In some embodiments, the first mounting portion 14 comprises one or morefixtures (e.g., mounting faces or blocks) or cut-outs into which thedirectional reference system 16 may be fitted. In various embodiments,the directional reference system 16 is releasably secured to the firstmounting portion 14. For example, the first mounting portion 14 mayinclude one or more straps, clamps, snaps, latches, threaded posts orsockets, etc., for mounting the directional reference system 16. Inaddition, the directional reference system 16 may include one or moremounting features which are configured to be coupled to correspondingmating features on the first mounting portion 14. In other embodiments,the directional reference system 16 and the first mounting portion 14may be generally permanently coupled (e.g., welded or glued together).In certain configurations, the first mounting portion 14 comprises orforms a part of a shelf structure which is mounted on or above the baseportion 12.

The first mounting portion 14 may also include one or more ports (notshown) (e.g., electrical ports) for operatively coupling the directionalreference system 16 to the apparatus 10. For example, the ports mayenable electrical communication between the directional reference system16 and the apparatus 10 or components thereof. In certain otherembodiments, the directional reference system 16 is not in directcommunication with or otherwise operatively coupled to the apparatus 10but is in communication with one or more systems or subsystemsphysically separate from the apparatus 10. Such systems or subsystemsmay themselves be in communication with the apparatus 10 or componentsthereof.

In certain embodiments, the at least one directional reference system 16comprises at least one signal receiver of a global positioning system(GPS). For example, the at least one signal receiver may comprise afirst antenna 22 and a second antenna 24 spaced apart from the firstantenna 22. In certain such embodiments, the first antenna 22 and thesecond antenna 24 define a line 26 from the first antenna 22 to thesecond antenna 24. In certain embodiments more than two antennae may beused. In certain embodiments, the at least one signal receiver furthercomprises a processor (not shown) configured to receive signals from thefirst and second antennae 22, 24 and to determine an orientation of theline 26 with respect to the reference direction 18. For example, theprocessor may be configured to determine an attitude or azimuth of thedirectional reference system 16 with respect to the reference direction18. In certain embodiments, the attitude or azimuth determination isrelatively precise. For example, the determination can be within about0.2 degrees in some embodiments. In other embodiments the determinationmay be more or less precise. In certain embodiments, the first mountingportion 14 comprises a first antenna mount 28 to be mechanically coupledto the first antenna 22 and a second antenna mount 29 to be mechanicallycoupled to the second antenna 24.

In certain other embodiments, the at least one signal receiver may be anon-GPS signal receiver. For example, the at least one signal receivermay be a signal receiver of another satellite navigation system (e.g.,GLONASS), or some non-satellite based navigation or positioning system.As shown, the directional reference system 16, the components thereof,and the base portion 12 may form one physically integral unit (e.g., thegenerally rectangular unit of FIG. 2). In certain other embodiments, thedirectional reference system 16 comprises one or more physicallyseparate units, each independently mounted on the base portion 12. Forexample, in one embodiment, the first antenna 22 forms a first unit tobe mounted to the first antenna mount 28 and the second antenna 24 formsa second unit to be mounted to the second antennae mount 29 andphysically separate from the first unit.

In some embodiments, surveying methods (e.g., optical sighting methodssuch as fore-sighting) may be used an alternative method of definingdetermining or defining the orientation of the platform or a line on theplatform with respect to the reference direction 18. In suchembodiments, a directional reference system 16 may not be employed andanother device, such as a sighting or other surveying device, forexample, may be used to determine the orientation (e.g., the direction19 of the apparatus 10) of the platform or a line thereon (e.g., a linecorresponding to the direction 19 of the apparatus 10) with respect tothe reference direction 18. Land-surveying techniques (e.g.,fore-sighting) may thus be used to determine an initial orientation(e.g., attitude and/or azimuth) of the apparatus 10 or a portion thereofwith respect to the reference direction 18. In certain embodiments, theorientation may be determined by optically sighting to a referenceobject or point at a known location with respect to the location of theapparatus 10 (e.g., an oil rig location). The first mounting portion 14of such embodiments may be configured to receive and accommodate thesurveying device (e.g., a sighting device). The first mounting portion14 may comprise features described above with respect to FIG. 2, forexample (e.g., one or more cut-outs, clamps, snaps, latches, threadedposts or sockets, etc.), but such features are generally configured tomount the surveying device instead of the directional reference system16. Data indicative of the initial orientation of the platform (e.g.,the direction 19 of the platform with respect to the reference direction18) may then be transmitted to the survey tool 30. In one embodiment,the data may be manually entered by an operator into a computing systemin communication with the survey tool 30 and then be transmitted to thetool 30 (e.g., wirelessly). Because the survey tool 30 of certainembodiments is mounted in a predetermined orientation with respect tothe apparatus 10 (e.g., parallel with the apparatus 10), the orientationof the survey tool 30 can be determined in accordance with embodimentsdescribed herein.

The second mounting portion 20 of certain embodiments comprises an areaof the base portion 12 on which the survey tool 30 is mounted. Forexample, the second mounting portion 20 may comprise the area or surface21 of the base portion 12. In some embodiments, the second mountingportion 20 comprises one or more fixtures or cut-outs into which thesurvey tool 30 may be fitted. In various embodiments, the survey tool 30is releasably secured to the second mounting portion 20. In certainembodiments, the second mounting portion 20 comprises one or moremounting faces or blocks. For example, the mounting faces may be similarto the mounting faces 46 and can extend from the base portion 12 and bepositioned on the apparatus 10 such that the survey tool 30 abutsagainst one or more surfaces of the mounting faces, thereby securingand/or limiting the movement of the survey tool 30 along the baseportion 12 in one or more directions. The mounting faces may compriseblocks (e.g., rectangular, cylindrical, triangular, etc. shaped blocks),sheets, and the like. In certain embodiments, the first mounting portion14, the third mounting portion 44 (FIG. 4), and/or the fourth mountingportion 53 (FIG. 4) can comprise mounting faces similar to the mountingfaces 46 of the second mounting portion 20 and which are configured tosecure and/or limit the movement of the directional reference system 16,the inertial navigation system 42, and the computing system 52,respectively. The apparatus 10 of FIG. 4 includes mounting faces 46 onone side of the survey tool 30. Other configurations are possible. Forexample, in one embodiment, there are mounting faces 46 on the oppositeside of the survey tool 30 and/or on each end of the survey tool 30.

In various embodiments, the second mounting portion 20 may include oneor more straps, clamps, snaps, latches, threaded posts or sockets, etc.,for mounting the survey tool 30. In addition, the survey tool 30 mayinclude one or more mating features configured to be coupled tocorresponding mating features on the second mounting portion 20. In someembodiments, the second mounting portion 20 comprises one or moresecuring elements (e.g., straps, clamps, etc.) positioned along thecasing of the survey tool 30 when the survey tool 30 is mounted. Incertain embodiments, the securing elements are positioned along one orboth of the long sides of the casing of the survey tool 30, at one orboth of the two ends of the casing of survey tool 30, or a combinationthereof. In various other embodiments, the securing elements arepositioned along only one side, along one or more of the ends of thecasing of the survey tool 30, or beneath or above the casing of thesurvey tool 30. In certain embodiments, the second mounting portion 20comprises or forms a part of a shelf structure which is mounted on orabove the base portion 12. For example, in one embodiment, the firstmounting portion 14 and the second mounting portion 20 each compriseseparate shelf structures and form a multi-leveled shelf structure on orover the base portion 12.

The second mounting portion 20 may also include one or more ports (e.g.,electrical ports) for operatively coupling the survey tool 30 to theapparatus 10. For example, the ports may enable electrical communicationbetween the survey tool 30 and the apparatus 10 or components thereof.In certain other embodiments, the survey tool 30 is not in directcommunication or otherwise operatively coupled to the apparatus 10, butis in communication with one or more systems or subsystems physicallyseparate from the apparatus 10. Such systems or subsystems maythemselves be in communication with the apparatus 10 or componentsthereof.

The survey tool 30 of certain embodiments can comprises various sensorsand computing hardware such that it can make use of various measuredquantities such as one or more of acceleration, magnetic field, andangular rate to determine the orientation of the survey tool 30 and ofthe wellbore with respect to a reference vector such as the Earth'sgravitational field, magnetic field, or rotation vector. In certainembodiments, the survey tool 30 is a dedicated survey instrument while,in other embodiments, the survey tool 30 is a measurement while drilling(MWD) or logging while drilling (LWD) instrumentation pack which may becoupled to a rotary steerable drilling tool, for example.

Because the line 26 between the two antennae 22, 24 may be generallyaligned with a direction 19 of the apparatus 10, or the orientation ofthe line 26 with respect to the apparatus 10 may otherwise be known, theline 26 may define, correspond to, or be used as the orientation (e.g.,direction 19) of the apparatus 10 with respect to the referencedirection 18. In FIG. 2, for example, the line 26 is shown rotated withrespect to the reference direction 18 (e.g., true north) by angle A. Theangle A may define or be characterized as the angle (e.g., azimuthangle) of the apparatus 10 with respect to the reference direction 18.Moreover, because the survey tool 30 can be aligned with respect to theline 26, the angle A can therefore also correspond to the direction(e.g., azimuth direction) of the survey tool 30 with respect to thereference direction 18. The angle A can thus be transmitted (e.g., aselectronic data) to the survey tool 30 for the initialization of thesurvey tool 30.

Loss of satellite telemetry to and/or detected by the directionalreference system 16 can arise in some conditions. Such loss can occur,for example, due to shielding of one or more of the GPS antennae fromone or more of the satellites by a derrick or other equipment on a rig.In addition, relatively unfavorable positioning of the satellites thatare in view of the platform can lead to a loss of precision in theorientation (e.g., attitude and/or azimuth) determination process. Thisloss of precision may be referred to as the geometric dilution ofprecision, for example. FIG. 3 schematically illustrates the apparatus10 according to certain embodiments described herein in a first location32 on a drilling rig 35 having a relatively clear communication pathbetween the antennae 22, 24 and the GPS satellites 36, 38, and in asecond location 34 at which one or more of the antennae 22, 24 areshielded from communication with one or more GPS satellites 36, 38 bythe derrick 31. As illustrated by the dotted lines, the apparatus 10 towhich the survey tool 30 is to be mounted for initialization is in clearview of the satellites 36, 38 in the first location 32 when spaced fromthe derrick 31 by a first distance 40. As such, a relatively clearcommunication path may exist between the antennae 22, 24 and thesatellites 36, 38. On the other hand, when located directly under thederrick 31 in the second position 34, the derrick 31 may block orotherwise interfere with communications from the satellites 36, 38 tothe antennae 22, 24, and there may no longer be a relatively clearcommunication path between the antennae 22, 24 and the satellites 36,38. As such, satellite telemetry to and/or detected by the directionalreference system 16 may be interrupted. In the example configuration ofFIG. 3, communications from the satellites 36, 38 to the antennae may besimilarly interrupted when the apparatus 10 is in other positions, suchas when the apparatus 10 is positioned to the left of the derrick 31.The distance 40 may generally be selected so as to ensure a relativelyclear communication path between the antennae 22, 24 and the satellites36, 38. For example, the distance 40 may range from 5 to 10 meters incertain embodiments. In other embodiments, the distance 40 can be lessthan 5 meters or greater than 10 meters.

It can be beneficial to have the capability to move the apparatus 10(e.g., along the surface of a rig) between the first location 32 wherethe effect of signal shielding is small (e.g., where the apparatus 10 isspaced apart from the drilling derrick 31) and the second location 34,where the survey tool 30 may be inserted into the wellbore but where thesatellite telemetry may be compromised. In certain embodiments, anorientation of the directional reference system 16 and/or survey tool 30may be accurately obtained at the first location 32 without substantialobstruction or other interference from the derrick 31, or from othersources. In addition, it is desirable to be able to keep track of therelative orientation of the apparatus 10 or components thereof as itmoves from the first location 32 to the second location 34. As such,deviations from the at the first location 32 may be tracked while theapparatus 10 is moved to the second location 34, thereby maintaining anup-to-date orientation (e.g., attitude, azimuth, and/or heading) of theapparatus and components thereof during movement. As described herein,an inertial navigation system may be used for such purposes.

FIG. 4 schematically illustrates an example apparatus 10 in accordancewith certain embodiments described herein. The apparatus 10 of certainembodiments includes a third mounting portion 44 mechanically coupled tothe base portion 12. The third mounting portion 44 is configured to bemechanically coupled to at least one inertial navigation system 42. Incertain embodiments, the third mounting portion 44 comprises an area ofthe base portion 12 on which the inertial navigation system 42 ismounted. In some embodiments, the third mounting portion 44 comprisesone or more fixtures or cut-outs into which the inertial navigationsystem 42 may be fitted. In various embodiments, the inertial navigationsystem 42 is releasably secured to the third mounting portion 44. Forexample, the third mounting portion 44 may include one or more straps,clamps, snaps, latches, or threads, etc. for mounting the inertialnavigation system 42. In addition, the inertial navigation system 42 mayinclude one or more mating features configured to be coupled tocorresponding mating features on the third mounting portion 44. In otherembodiments, the inertial navigation system 42 and the third mountingportion 44 may be generally permanently coupled (e.g., welded or gluedtogether). In certain embodiments, the third mounting portion 44comprises or forms a part of a shelf structure which is mounted on orabove the base portion 12. For example, in one embodiment, the thirdmounting portion 44 and one or more of the first mounting portion 14 andthe second mounting portion 20 may each comprise separate shelves andform a multi-leveled shelf structure on or over the base portion 12.

The third mounting portion 44 may also include one or more ports (e.g.,electrical ports) for operatively coupling the inertial navigationsystem 42 to the apparatus 10. For example, the ports may enableelectrical communication between the inertial navigation system 42 andthe apparatus 10 or components thereof. In certain other embodiments,the inertial navigation system 42 is not in direct communication orotherwise operatively coupled to the apparatus 10, but is incommunication with one or more systems or subsystems physically separatefrom the apparatus 10. Such systems or subsystems may themselves be incommunication with the apparatus 10 or components thereof.

The inertial navigation system 42 generally provides the capability ofmaintaining the heading or orientation information obtained at the firstlocation 32 while the apparatus 10 is moved from the first location 32(e.g., on a rig from the first location 32 to the second location 34).The inertial navigation system 42 may comprise an attitude and headingreference system (AHRS), for example, and may be used to keep track ofthe orientation of the apparatus 10 and components thereon (e.g.,attitude and/or azimuth) during movement of the apparatus 10 (e.g., fromthe first location 32 to the second location 34 of FIG. 3). For example,the inertial navigation system 42 may keep track of the orientation(e.g., attitude, azimuth, and/or heading) during movement of theapparatus 10 should the performance of the directional reference system16 become compromised (e.g., the antennae of a GPS system are obscuredfrom the satellite by the derrick 31 on a rig) or cannot be used todetermine the orientation of the apparatus at the well head of thewellbore. In other embodiments, other types of inertial navigationsystems, such as a full inertial navigation system (INS) may be used. Insome embodiments, the directional reference system 16 or componentsthereof and the inertial navigation system 42 may be integrated into asingle unit (e.g., a GPS/AHRS unit).

FIG. 5 schematically illustrates a top view of an apparatus 10 includingan integrated GPS/AHRS unit 43 in accordance with certain embodimentsdescribed herein. Referring again to FIG. 4, the inertial navigationsystem 42 may comprise a processor and one or more motion sensors (e.g.,accelerometers) positioned within the GPS/AHRS unit 43 and may beconfigured to generally continuously calculate the position,orientation, and/or velocity of the apparatus 10 as it is moved.

As shown in FIG. 4, the second mounting portion 20 of certainembodiments may comprise one or more mounting faces 46 which aredescribed in detail above with respect to FIG. 2.

The apparatus 10 further comprises at least one leveler 48 configured tolevel the apparatus 10 with respect to the Earth (e.g., to besubstantially perpendicular to the direction of gravity). The at leastone leveler 48 may comprise a set of one or more adjustable supports,for example. Various adjustment mechanisms are possible. For example, inone embodiment, the leveler 48 comprised a retractable portion (e.g., athreaded rod) which can be used to lengthen or shorten the leveler 48(e.g., by extending from and retracting into the base portion 12). Inanother embodiment, the leveler comprises an expandable portion (e.g., aballoon or other finable member) which can be inflated and deflated toadjust the length of the leveler to level the apparatus 10 with respectto the Earth. The apparatus 10 of FIG. 4 comprises three levelers 48(one of which is not shown) shaped as cylindrical support posts. Oneleveler 48 is attached to the underside of one corner of the baseportion 12, one leveler 48 is attached to the underside of a neighboringcorner of the base portion 12, and one leveler 48 (not shown) isattached to the center of a side between two other corners of the baseportion 12. In some embodiments, the at least one leveler 48 comprisesan elongate leg portion attached to the base portion 12 and a footportion which contacts the surface beneath the apparatus 100. The footportion of certain embodiments is generally widened with respect to theleg portion and may be attached to the bottom of the leg portion. In oneembodiment, there are four levelers 48, each attached to the undersideof one of the four corners of the base portion 12. In anotherembodiment, the levelers 48 comprise a set of elongate members eachattached to and extending laterally from a side of the base portion 12,and extending downwards to make contact with the surface beneath theapparatus 10. In yet other embodiments, the at least one levelercomprises one or more rails extending along the underside of the baseportion 12. In other embodiments, there may be one leveler 48, twolevelers 48, or more than three levelers 48 and/or the levelers 48 maybe shaped or configured differently (e.g., as rectangular posts, blocks,hemispherical protrusions, etc.).

In addition, the apparatus 10 may further comprise at least one leveldetector 50 configured to generate a signal indicative of the level ortilt of the apparatus 10 with respect to the Earth. In certain suchembodiments, the at least one leveler 48 is configured to level theapparatus 10 with respect to the Earth in response to the signal fromthe at least one level detector 50. For example, the level detector 50may comprise a bubble-type level detector, or some other type of leveldetector. In certain embodiments, the apparatus 10 may include one ormore supports which are not adjustable. In certain other embodiments(e.g., where the apparatus 100 does not include a leveler 48), thesignal from the at least one level detector 50 may be used to adjustcomputations, such as computations regarding the orientation of theapparatus 10, components thereof (e.g., the directional reference system16), or the survey tool 30. For example, the signal may be used tocompensate for any level differences between the apparatus 10 and theEarth in such computations. In general, the at least one level detector50, in conjunction with the at least one leveler 48 can be configured todetect tilt of the apparatus 10 and physically level the apparatus 10 inresponse to such tilt.

In certain embodiments, the apparatus 10 further comprises at least onemember (not shown) movably coupled to a portion of the apparatus 10 andconfigured to allow the apparatus 10 to move along a surface beneath theapparatus 10. The surface may be the Earth's surface, a rig surface,etc. In certain embodiments, the at least one member comprises at leastone wheel configured to rotate about at least one axis. In otherembodiments, the at least one member may comprise a tread, ski, or othermechanism configured to allow for movement of the apparatus 10 along thesurface. For example, in one embodiment the apparatus 10 comprises fourwith each wheel positioned near a corresponding one of the four cornersof the base portion 12. The at least one member may beextendable/retractable such that it can be extended towards the surface(e.g., away from the base portion 12) for use and can be retracted awayfrom the surface (e.g., towards the base portion 12) when the at leastone member is not in use. For example, in one embodiment, the at leastone member comprises a set of wheels which can be extended from a firstposition in which the wheels are not in contact with the surface to asecond position in which the wheels are in contact with the surface formoving the apparatus 10 along the surface. The wheels can then be raisedfrom the second position back to the first position, such as when theapparatus 10 has reached the desired destination. The raising of thewheels can allow for relatively improved stability of the apparatus 10on the surface in certain embodiments (e.g., while survey tool is beinginitialized). In other embodiments, the at least one member is notretractable and is in continuous contact with the surface. In variousconfigurations, generally any number of members (e.g., 1, 2, 3, 4, 5, ormore) may be employed.

In certain embodiments, the apparatus 10 further comprises a computingsystem 52. In certain embodiments, the computer may be in communicationwith the directional reference system 16 (e.g., as indicated by arrow47), the inertial navigation system 42 (e.g., as indicated by arrow 45),and/or the survey tool 30 (e.g., as indicated by arrow 49). For example,the computing system 52 may receive data indicative of the orientationof the apparatus 10 with respect to the reference direction 18 from thedirectional reference system 16. The computing system 52 may alsoreceive information from the inertial navigation system 42, such asinformation regarding the position, orientation, and/or velocity of theapparatus 10 as it moves along the surface beneath the apparatus 10. Thecomputing system 52 may further be configured to process the informationfrom the directional reference system 16 and/or the inertial navigationsystem 42 to determine an initial orientation of the survey tool 30. Thecomputing system 52 may further be configured to transmit suchinformation to the survey tool 30 in some embodiments. In otherembodiments, the computing system 52 may transmit the data from thedirectional reference system 16 and/or the inertial navigation 42directly to the survey tool 30 for at least some of the processinginstead of performing the processing of the data itself. In someembodiments, there is no computing system 52, and the survey tool 30receives the data directly from the directional reference system 16 andthe inertial navigation system 42 and processes the data itself.

The apparatus 10 may further comprise a fourth mounting portion 53. Thefourth mounting portion 53 comprises an area of the base portion 12 onwhich the computing system 52 is mounted. In some embodiments, thefourth mounting portion 53 comprises one or more cut-outs or fixturesonto which the computing system 52 may be fitted. In variousembodiments, the computing system 52 is releasably secured to the fourthmounting portion 53. For example, the fourth mounting portion 53 mayinclude one or more straps, clamps, snaps, latches, or threads, etc. formounting the computing system 52. In addition, the computing system 52may include one or more mating features configured to be coupled tocorresponding mating features on the fourth mounting portion 53. Inother embodiments, the computing system 52 and the fourth mountingportion 53 may be generally permanently coupled (e.g., welded or gluedtogether). In certain embodiments, the fourth mounting portion 53comprises or forms a part of a shelf structure which is mounted on orabove the base portion 12. For example, in one embodiment, the fourthmounting portion 53 and one or more of the first mounting portion 14,the second mounting portion 20, and the third mounting portion 44 mayeach comprise separate shelves and form a multi-leveled shelf structureon or over the base portion 12.

The fourth mounting portion 53 may also include one or more ports (e.g.,electrical ports) for operatively coupling the computing system 52 tothe apparatus 10. For example, the ports may enable electricalcommunication between the computing system 52 and the apparatus 10 orcomponents thereof.

In certain embodiments, the apparatus 10 further comprises a toolpositioning element 56. FIGS. 6A-6C schematically illustrate top, frontand right side views, respectively, of an apparatus 10 including a toolpositioning element 56. The tool positioning element 56 can beconfigured to controllably move the wellbore survey tool 30 between afirst position relative to the apparatus 10 and a second positionrelative to the apparatus 10. In certain embodiments, the first positionis horizontal with respect to the base portion 12 and the secondposition is vertical with respect to the base portion 12. In otherembodiments, the survey tool 30 may be positioned at an angle relativeto the base portion 12 in one or more of the first and second positions.In certain embodiments, the tool positioning element 56 comprises amotorized system such as a motor drive 60. The tool positioning element56 may be configured to rotate the surface 21 of the second mountingportion 20 to which the survey tool 30 can be coupled and which can berotated (e.g., using the motorized drive 60 or another motorized system)with respect to the base portion 12 from horizontal to vertical so as tomove the survey tool 30 between the first position and the secondposition. In other embodiments, the tool positioning element 56comprises a pulley system (e.g., a motorized pulley system) for liftingand lowering the survey tool 30 between the first position and secondposition, or some other mechanism for moving the survey tool 30.

FIG. 6D schematically illustrates a partial perspective view of anapparatus 10 including a tool positioning element 56 during positioningof a survey tool 30 in accordance with certain embodiments describedherein. The drive motor 60 of the apparatus 10 of FIG. 6D is visiblethrough the base portion 12 for the purposes of illustration. Asindicated by the directional arrow 25, the tool positioning element 56is movable between a first (e.g., horizontal) position and a second(e.g., vertical position). The tool positioning element 56 may, incertain embodiments, controllably move or rotate the survey tool 30 ininclination while it is attached or otherwise coupled to the apparatus10. The survey tool 30 is shown in FIG. 6D during movement of the surveytool 30 by the positioning element 56 between the first and secondpositions such that the survey tool 30 is currently positioned at anangle B with respect to surface 13 of the apparatus 10. As shown, thedrive motor 60 of the positioning element 56 is configured tocontrollably move the surface 21 to which the survey tool 30 can begenerally rigidly attached about the axis 66 between the first andsecond position.

In one example scenario, the tool positioning element moves the surveytool 30 is mounted to the apparatus 10 in a generally verticalorientation, while the surface 21 is positioned by the tool positioningelement 56 in a generally vertical orientation with respect to thesurface 13 of the base portion 12. The surface 21 and survey tool 30mounted thereon are then rotated by the positioning element 56 such thatthe surface 21 and survey tool 30 are generally horizontal or flush withrespect to the surface 13 of the base portion 12. The survey tool 30 maybe initialized using the initialization process described herein whilein the horizontal position. The survey tool 30 may then be rotated backto the vertical position by the tool positioning element 56 and thendisconnected or un-mounted from the apparatus 10 at which point thesurvey tool 30 may be supported by a wire line 58, for example andlowered into the well bore.

In other embodiments, the survey tool 30 is not rotated to horizontal,but is rotated to some other angle with respect to the apparatus 10(e.g., 15 degrees, 30 degrees, 45 degrees, 60 degrees, etc.). Inaddition, the survey tool 30 may not be rotated to a complete verticalposition, but to some other angle with respect to the apparatus 10. Inother embodiments, the apparatus 10 does not include a positioningelement 56. In such embodiments, the survey tool 30 may be mountedgenerally in the orientation (e.g., vertical with respect to the surface13 of the apparatus 10) in which the apparatus 10 will be deployed tothe well bore. In addition, the positioning element 56 may be positionedor mounted differently on the apparatus 10. For example, the motor drive60 and corresponding axis 66 are shown positioned generally in themiddle cut-out portion 23 in FIG. 6D. As such, when the survey tool 30is positioned in the vertical position, half of the survey tool 30 ispositioned substantially above the base portion 12 and the other half ofthe survey tool 30 is positioned above the base portion 12. In otherembodiments, the corresponding motor drive 60 axis 66 may be positioneddifferently, such as generally at one end of the cut-out portion 23. Insome such cases, the positioning element 56 may rotate the survey tool30 generally from a horizontal position to a vertical position in whicha survey tool 30 or a substantial portion thereof is rotated under thebase portion 12. In other such cases, the positioning element may rotatethe survey tool 30 generally from a horizontal position to a verticalposition in which a survey tool 30 or a substantial portion thereof isrotated above the base portion 12.

It is desirable to move (e.g., rotate) the tool at a relatively low rate(e.g., within the rate limits of the gyroscopes on the survey tool 30).Certain embodiments advantageously avoid turning of the survey tool 30undesirably high turn rates which exceed the maximum rates which can bemeasured by one or more rotation sensors (e.g., gyroscopes) of thesurvey tool 30. Under such undesirable conditions, the orientation data(e.g., directional reference data) stored in the survey tool 30 can belost and subsequent orientation (e.g., attitude and/or azimuth)processing will be in error. By controllably moving the survey tool 30(e.g., using the drive motor 60 about the axis 66), the tool positioningelement 56 may, in certain embodiments, avoid saturation of sensors ofthe survey tool 30 and thereby allow the survey tool 30 to continue tokeep track of its rotation as it is moved.

In an example use scenario, the apparatus 10 can be location at aposition at which the directional reference system 16 is operational andthe reference direction 18 may be determined using the directionalreference system 16 (e.g., a GPS signal receiver). The apparatus 10 maythen be moved physically to the well head of the wellbore (e.g., usingthe at least one member movably coupled to a portion of the apparatus10) with the orientation or directional reference being maintained,monitored, or detected by the inertial navigation system 42 (e.g., anAHRS unit) while the apparatus 10 is moved. In certain embodiments, thismovement occurs over a relatively short period of time (e.g., on theorder of several minutes). Once positioned at the well head, the surveytool 30 may be placed into a designated position (e.g., to the secondmounting portion 20) and clamped to the apparatus 10. The orientationdata (e.g., attitude, azimuth and/or heading data) may then betransmitted from the inertial navigation system 42 (e.g., an AHRS) tothe wellbore survey tool 30 to initialize the survey tool 30. Forexample, the orientation data may be transmitted to an inertial systemwithin the survey tool 30 via the computing system 52 or, alternatively,directly to the wellbore survey tool 30. In certain other embodiments,the survey tool 30 is mounted on to the apparatus 10 while the apparatus10 is moved from the first position to the second position.

FIG. 7 schematically illustrates an embodiment in which the directionalreference system 16 is mounted directly on the wellbore survey tool 30in accordance with certain embodiments described herein. The directionalreference system 16 comprises at least one signal receiver of a globalpositioning system (GPS) which can include a first antenna 22 and asecond antenna 24 spaced apart and defining a line 26 from the firstantenna 22 to the second antenna 24. In certain embodiments, the surveytool 30 comprises a processor 54 configured to receive signals from thefirst and second antennae 22, 24 and to determine an orientation of theline 26 with respect to the reference direction in response to thesignals. Because a processor 54 of the survey tool 30 may be usedinstead of a dedicated processor of the directional reference system 16,hardware costs may thereby be reduced. In addition, because thedirectional reference system 16 may be directly mounted on the surveytool 30, there may be less calibration inaccuracy due to possiblemisalignments in the orientation of the directional reference system 16with respect to the survey tool 30. In other embodiments, thedirectional reference system 16 comprises a processor which is used todetermine the orientation and a processor of the survey tool 30 is notused. For example, the processor 53 may be configured to determine anorientation (e.g., attitude and/or azimuth) of the directional referencesystem with respect to the reference direction.

Where the directional reference system 16 (e.g., a GPS signal receivercomprising the two or more antennae 22, 24) is mounted on or within thesurvey tool 30 itself, as illustrated in FIG. 7, the survey tool 30itself can be mounted relatively rigidly on the drilling rig (e.g., in ahorizontal or other non-vertical orientation) to conduct theinitialization process (e.g., initial attitude and headingdetermination). For example, the orientation (e.g., attitude)determination may be made using measurements of the phase difference inthe satellite carrier signals (e.g., between the antennae 22, 24). Sucha determination may be made by computation by the processor 54 withinthe survey tool 30, for example. This information may again be used todefine the initial attitude of the survey tool 30 prior to engaging orinitializing a continuous survey mode. The attitude data (e.g., dataderived from GPS data from the directional reference system 16) can formthe initial conditions for the gyro measurement integration process,which allows for tracking of the attitude of the survey tool 30 afterthe initialization.

In certain embodiments, the apparatus 10 further comprises at least oneof the at least one directional reference system 16 and the at least oneinertial navigation system 42. In certain embodiments in which theapparatus comprises the at least one directional reference system 16,the apparatus 10 further comprises a mounting portion (e.g., one or moreportions of the base portion 12, the first mounting portion 14, thesecond mounting portion 20, the third mounting portion 44, and thefourth mounting portion 53) mechanically coupled to the at least onedirectional reference system 16 and configured to be mechanicallycoupled to the wellbore survey tool 30 while the wellbore survey tool 30is outside a wellbore such that the wellbore survey tool 30 has apredetermined orientation with respect to the at least one directionalreference system 16 while the wellbore survey tool 30 is outside thewellbore. The mounting portion may be further configured to bemechanically decoupled from the wellbore survey tool 30 while thewellbore survey tool 30 is within the wellbore. The apparatus 10 mayfurther comprise a support structure configured to allow the apparatusto move along a surface beneath the apparatus while the wellbore surveytool 30 is transported outside the wellbore. For example, in certainembodiments, the support structure may comprise one or more of the baseportion 12, the at least one member movably coupled to a portion of theapparatus 10, the at least one leveler 48, or portions thereof, asdescribed herein.

Embodiments described herein may further be used to provide a relativelylong term attitude reference on the drilling rig. As discussed, afterinitialization of the survey tool 30 according to embodiments describedherein, the survey tool 30 may be deployed into the wellbore and used toconduct a survey (e.g., in continuous survey mode). In certain cases,the survey tool 30 may have been initialized accurately according toembodiments described herein prior to deployment, but calibration errorsmay accumulate during operation, thereby causing “drift.” Suchcalibration errors may be acceptable under certain circumstances (e.g.,where the drift of less than about 10%). However, relatively largecalibration errors can be problematic and it can be desirable to measuresuch errors. In certain embodiments, after withdrawal of the survey tool30 from the wellbore, the survey tool 30 orientation (e.g., attitude)determined by the survey tool 30 can be compared to a referenceorientation (e.g., attitude) determined by the apparatus 10 to canprovide a post-survey check on the calibration or amount of drift of thesurvey tool 30. For example, the survey tool 30 may be mounted to theapparatus 10 following its withdrawal from the wellbore and readings ofthe orientation (e.g., attitude) of the survey tool 30 from the surveytool 30 may be compared to readings of the orientation (e.g., attitude)from the directional reference system 16. In certain other embodiments,the orientation readings from the survey tool 30 may be compared toreadings from the orientation of the inertial navigation system 42, orfrom an integrated device such as the GPS/AHRS 43 of FIG. 5. Differencesin orientation determined from such a comparison may correspond tocalibration errors or “drift.” This general process may be described asa quality control (QC) check on the ‘health’ of the survey tool 30, forexample.

FIG. 8 is a flow diagram illustrating an example wellbore survey tool 30initialization process 100 in accordance with certain embodimentsdescribed herein. While the flow diagram 100 is described herein byreference to the apparatus 10 schematically illustrated by FIGS. 2-6,other apparatus described herein may also be used (e.g., the apparatus400 of FIG. 11). At operational block 102, the survey tool 30 can besuspended above the base portion of the apparatus 10, such as by awire-line, for example. The apparatus 10 may then be leveled atoperational block 104 by adjusting one or more of the at least onelevelers 48 (e.g., an adjustable support), for example.

At operational block 106, the directional reference system 16 (e.g., GPSreceiver, integrated GPS/AHRS) and/or inertial navigation system 42 maybe initiated and may generate one or more signals indicative of theorientation (e.g., the attitude, azimuth, and/or heading) of theapparatus 10. At operational block 108, the apparatus 10 may be moved tothe well head of the wellbore. This movement of the apparatus 10 may beperformed in situations where the apparatus 10 has initially beenpositioned away from the wellbore, to avoid interference from a derrick,for example. The survey tool 30 may be lowered and attached to theapparatus 10 (e.g., clamped to the second mounting portion 20) atoperational block 110. The survey tool 30 may be rotated to thehorizontal (e.g., with respect to the base portion 12 of the apparatus10) at operational block 112 and power may be supplied to the surveytool 30 at operational block 114.

At operational block 116, the orientation (e.g., attitude, azimuth,and/or heading) data from the directional reference system 16, inertialnavigation system 42, or both, may be transferred to the survey tool 30.In some embodiments, an angular rate matching process (e.g., using anangular rate matching filter) as described below is employed. The toolmay be switched to continuous survey mode at operational block 118, andmoved (e.g., rotated using the tool positioning element 56) to vertical(e.g., with respect to the apparatus 10) at a controlled rate atoperational block 120. The survey tool 30 can be detached from theapparatus 10 while still being supported (e.g., by a wire-line) atoperational block 122 and raised above the apparatus 10 at operationalblock 124. The survey tool 30 may be lowered into the top of thewellbore at operational block 126 and continuous surveying may beenabled at operational block 128.

FIG. 9 is a flowchart of an example method 200 of initializing awellbore survey tool 30 in accordance with certain embodiments describedherein. At operational block 202, the method 200 includes positioning awellbore survey tool 30 at a predetermined orientation relative to adirectional reference system 16. For example, the wellbore survey tool30 may be positioned substantially parallel to the directional referencesystem 16 in certain embodiments. While the method 200 is describedherein by reference to the apparatus 10 described with respect to FIGS.2-7, other apparatus described herein may be used (e.g., the apparatus400 of FIG. 11).

At operational block 204, the method 200 of certain embodiments furthercomprises generating a first signal indicative of an orientation of thedirectional reference system 16 with respect to a reference direction18. For example, the first signal may be generated by the directionalreference system 16, and the reference direction may be north. Themethod 200 may further comprise determining an initial orientation ofthe wellbore survey tool 30 with respect to the reference direction 18in response to the first signal at operational block 206. For example, acomputing system 52 of the apparatus 10 may receive the first signalfrom the directional reference system 16 and determine the orientationof the directional reference system 16 with respect to the referencedirection 18 in response to the first signal. In certain embodiments,because the wellbore survey tool 30 is positioned at a predeterminedorientation (e.g., parallel) relative to the directional referencesystem 16, the computing system 52 can also determine the initialorientation of the survey tool 30 with respect to the referencedirection 18.

At operational block 208, the method 200 further comprises moving thewellbore survey tool 30 from a first position to a second position afterdetermining the initial orientation of the wellbore survey tool 30. Forexample, the wellbore survey tool 30 may be substantially horizontalwith respect to the Earth when in the first position and the wellboresurvey tool 30 may be substantially vertical with respect to the Earthwhen in the second position. The tool positioning element 56, (e.g., amotorized system) can be used to controllably move the survey tool fromthe first position to the second position, as described herein.

In some embodiments, the method 200 may further comprise moving thewellbore survey tool 30 from a first location 32 to a second location 34(FIG. 3) after generating the first signal. The first location 32 may befarther from the wellbore than the second location 34. As describedherein, the directional reference system 16 may be able to accuratelydetermine the orientation of the directional reference system 16 withrespect to the reference direction 18 at the first location 32. Forexample, the directional reference system 16 may comprise a signalreceiver of a satellite navigation system which can communicate withsatellites of the satellite navigation system free from shielding orother interference from the derrick 31 at the first location 32, but notat the second location 34. The wellbore survey tool 30 may have a firstorientation with respect to the reference direction 18 when at the firstlocation 32 and a second orientation with respect to the referencedirection 18 when at the second location 34. For example, theorientation of the apparatus 10, and thus of the directional referencesystem 16 and the survey tool 30 coupled to the apparatus 10, may changein angle with respect to the reference direction 18 as the apparatus 10moves from the first location 32 to the second location 34.

The method 200 may further comprise generating a second signalindicative of a change in orientation between the first orientation andthe second orientation. For example, the computing system 52 may receivethe second signal from the inertial navigation system 42. In certainembodiments, the determining the initial orientation in the operationalblock 206 comprises determining the initial orientation of the wellboresurvey tool 30 with respect to the reference direction 18 in response tothe first signal and in response to the second signal. For example, thecomputing system 52 may determine the first orientation of thedirectional reference system 16 and thus the survey tool 30 at the firstlocation in response to the first signal. The computing system 52 maythen determine the change in orientation of the survey tool between thefirst orientation and the second orientation in response to the secondsignal. The computing system 52 may further process the first and secondsignals (e.g., add the change in orientation to the initial orientation)to determine the initial orientation of the survey tool 30 at the secondlocation.

C. Example Attitude Computation in the Survey Tool

In certain circumstances, the initial orientation data (e.g., referenceattitude data determined in accordance with embodiments describedherein) form the initial conditions for the gyro measurement integrationprocess which can keep track of survey tool 30 attitude while acontinuous survey mode of operation is maintained. During continuousperiods of operation (e.g., during continuous survey mode), the surveytool 30 may keep track of attitude (tool face, inclination and azimuth)using the integrated outputs of the gyroscopes. Tracking of the attitudemay involve solving the following equations to provide estimates oftool-face (α), inclination (I) and azimuth (A) angles:

α=α₀ +∫{dot over (α)}dt;  (Eq. 2)

I=I ₀ +∫İdt; and  (Eq. 3)

A=A ₀ +∫{dot over (A)}dt,  (Eq. 4)

where α₀, I₀ and A_(o) are the initial values of tool face, inclinationand azimuth, and {dot over (a)}, İ and {dot over (A)} are the estimatedrates of change of α, I and A which may be expressed as function of thegyro measurements (denoted G_(x), G_(y), and G_(z)) as follows:

$\begin{matrix}{{\overset{.}{\alpha} = {G_{z} + {\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)\cot \; I} - \frac{\Omega_{H}\cos \; A}{\sin \; I}}};} & \left( {{Eq}.\mspace{14mu} 5} \right) \\{{{\overset{.}{I} = {{{- G_{x}}\cos \; \alpha} + {G_{y}\sin \; \alpha} + {\Omega_{H}\sin}}};}{and}} & \left( {{Eq}.\mspace{14mu} 6} \right) \\{\overset{.}{A} = {{- \frac{\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)}{\sin \; I}} + {\Omega_{H}\cos \; A\; \cot \; I} - \Omega_{V}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

where Ω_(H) and Ω_(v) represent the horizontal and vertical componentsof Earth's rate. The initial value of the azimuth angle can be deriveddirectly from the GPS attitude estimation process. An initial value ofinclination may also be derived using the GPS measurements, or usingsurvey tool 30 accelerometer measurements (A_(x), A_(y), and A_(z)) andthe following equation:

$\begin{matrix}{I_{0} = {{\arctan\left\lbrack \frac{\sqrt{A_{x}^{2} + A_{y}^{2}}}{A_{z}} \right\rbrack}.}} & \left( {{Eq}.\mspace{14mu} 8} \right)\end{matrix}$

The initial value of inclination may also be determined using acombination of both satellite and accelerometer estimates. Tool-faceangle is initialized using accelerometer measurements as follows:

$\alpha_{0} = {{\arctan \left\lbrack \frac{- A_{x}}{- A_{y}} \right\rbrack}.}$

D. Example Alternative Method of Computing Attitude

In accordance with certain embodiments described herein, the use ofdirection cosines allows the tool orientation to be tracked generally atany attitude, such as when the tool is at or near vertical as occursduring tool pick-up and initial descent in the wellbore. This allows themethods of keeping track of tool-face angle and azimuth discussed in theprevious section, which may be relatively imprecise, to be avoided. Theuse of the quaternion attitude representation can provide an alternativein this situation.

The attitude of an alignment structure (e.g., the directional referencesystem 16) on the apparatus 10, such as on a platform (P) of theapparatus 10 with respect to the local geographic reference frame (R)(e.g., the reference direction 18), which may be determined from the GPSmeasurements, may be expressed in term of the direction cosine matrixC_(P) ^(R). The reference frame R can be generally defined by thedirections of true north and the local vertical. In certain otherconfigurations, other Earth fixed reference frames may be used. Theplatform (P) may comprise or form a part of the base portion 12, forexample. Given knowledge of the mounting orientation of the survey tool(T) 30 with respect to the alignment structure (e.g., the directionalreference system 16), which may also be expressed as a direction cosinematrix, C_(T) ^(P), the attitude of the survey tool 30 with respect tothe geographic reference frame (R) is given by the product of thesematrices, as follows:

C _(T) ^(R) =C _(P) ^(R) ·C _(T) ^(P)  (Eq. 9)

After switching to continuous survey mode, the survey tool 30 can keeptrack of tool attitude as it traverses the wellbore by solving theequation below. Expressing C=C_(T) ^(R) and the initial value derivedfrom the GPS measurements as C_(o),

C=C _(o) +∫Ċdt,  (Eq. 10)

where

$\begin{matrix}{\overset{.}{C} = {C \cdot \left\lbrack {\omega \times} \right\rbrack}} & \left( {{Eq}.\mspace{14mu} 11} \right) \\{\omega = {\begin{bmatrix}G_{x} \\G_{y} \\G_{z}\end{bmatrix} - {C^{T} \cdot \begin{bmatrix}\Omega_{H} \\0 \\\Omega_{V}\end{bmatrix}}}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$

Attitude information expressed in terms of tool-face, inclination andazimuth may be computed, from the elements of the direction cosinematrix:

${C = \begin{bmatrix}c_{11} & c_{12} & c_{13} \\c_{21} & c_{22} & c_{23} \\c_{31} & c_{32} & c_{33}\end{bmatrix}},$

which may also be expressed as function of these angles as follows:

                                        (Eq.  13) $C = \begin{bmatrix}{{\cos \; A\; \cos \; I\; \sin \; \alpha} + {\sin \; A\; \cos \; \alpha}} & {{\cos \; A\; \cos \; I\; \cos \; \alpha} - {\sin \; A\; \sin \; \alpha}} & {\cos \; A\; \sin \; I} \\{{\sin \; a\; \cos \; I\; \sin \; \alpha} - {\cos \; A\; \cos \; \alpha}} & {{\sin \; A\; \cos \; I\; \cos \; \alpha} + {\cos \; A\; \sin \; \alpha}} & {\sin \; A\; \sin \; I} \\{{- \sin}\; I\; \sin \; \alpha} & {{- \sin}\; I\; \cos \; \alpha} & {\cos \; I}\end{bmatrix}$

In certain embodiments, the tool-face, inclination and azimuth anglesmay be extracted using the following equations:

$\begin{matrix}{{\alpha = {\arctan \left\lbrack \frac{- c_{31}}{- c_{32}} \right\rbrack}};} & \left( {{Eq}.\mspace{14mu} 14} \right) \\{{I = {\arctan\left\lbrack \frac{\sqrt{c_{31}^{2} + c_{32}^{2}}}{c_{33}} \right\rbrack}};{and}} & \left( {{Eq}.\mspace{14mu} 15} \right) \\{A = {{\arctan \left\lbrack \frac{c_{23}}{c_{13}} \right\rbrack}.}} & \left( {{Eq}.\mspace{14mu} 16} \right)\end{matrix}$

For example, using the above equation for inclination for the situationwhere inclination approaches 90°, c₃₃ approaches zero and I may becomeindeterminate. In this case, inclination may be expressed as follows:

I=arccos [c ₃₃].  (Eq. 17)

For the situation where I passes through zero, the equations in α and Agenerally become indeterminate because both the numerator and thedenominator approach zero substantially simultaneously. Under suchconditions, alternative solutions for α and A can be based upon otherelements of the direction cosine matrix. For example, α and A can bedetermined as follows:

c ₁₁ +c ₂₂=sin(α+A)·(cos I+1);  (Eq. 18)

c ₂₁ −c ₁₂=cos(α+A)·(cos I+1),  (Eq. 19)

and the following expression for the sum of azimuth and tool face may bewritten:

$\begin{matrix}{{\alpha + A} = {{\arctan \left\lbrack \frac{c_{11} + c_{22}}{c_{21} - c_{12}} \right\rbrack}.}} & \left( {{Eq}.\mspace{14mu} 20} \right)\end{matrix}$

This quantity corresponds to the so-called gyro tool-face angle that iscurrently computed while the tool is at or near vertical.

Separate solutions for α and A may not be obtained when I=0 because bothgenerally become measures of angle about parallel axes (about thevertical), i.e. a degree of rotational freedom is lost. Either α or Amay be selected arbitrarily to satisfy some other condition while theunspecified angle is chosen to satisfy the above equation. To avoid‘jumps’ in the values of α or A between successive calculations when Iis in the region of zero, one approach would be to ‘freeze’ one angle, afor instance, at its current value and to calculate A in accordance withthe above equation. At the next iteration, A would be frozen and adetermined. The process of updating α or A alone at successiveiterations could generally continue until I is no longer close to zero.

E. Example Attitude Matching Filter for the Transfer of Orientation Data(e.g., Attitude and Heading Reference Data) to the Survey Tool

In certain embodiments, orientation (e.g., attitude) data extracted fromsatellite navigation techniques (e.g., using the directional referencesystem 16) can be combined with inertial system data (e.g., from theinertial navigation system 42). For example, a least-squares or Kalmanfiltering process can be used determine a relatively accurate estimate(e.g., a best estimate) of survey tool 30 orientation (e.g., attitude)prior to engaging/initializing the continuous survey mode. Data whichmay be determined while the survey tool 30 is at the surface includes:

(1) satellite based estimates of azimuth and inclination (e.g., usingthe directional reference system 16);

(2) estimates of inclination and high-side tool-face angle of the surveytool 30 using accelerometers of the survey tool 30;

(3) estimates of azimuth, inclination and tool-face angle of the surveytool 30 using sensors gyroscopes of the survey tool 30;

An example filtering process is provided herein. Embodiments describedherein include a Kalman filter formulation that may be used toinitialize the continuous survey process while the survey tool 30 is atthe surface. In certain embodiments, it may be assumed that the surveytool 30 provides measurement of acceleration along, and turn rate about,the three principal axes of the tool, denoted x, y and z. Whilecontinuous estimates of survey tool 30 orientation can be derived fromthe gyro measurements by a process of integration, it may further beassumed that the accelerometer measurements can provide a separate andindependent estimate of survey tool orientation with respect to thelocal vertical. Further, a satellite attitude determination process(e.g., using the directional reference system 16) provides estimates ofsurvey tool 30 azimuth during this period. Gyro, accelerometer and GPSbased attitude estimates can be combined using a Kalman filter asdescribed below. In addition to providing initial estimates of toolorientation (e.g., attitude), the filtering process may also be used toform estimates of any residual gyro biases and mass unbalance.

System Equations

During periods where the survey tool 30 is in continuous mode, the toolkeeps track of attitude (e.g., tool face, inclination and azimuth) usingthe integrated outputs of the gyroscopes. This may be achieved bysolving the following equations to provide estimates of tool face (α),inclination (I) and azimuth (A) angles directly. For example, thesevalues may be expressed as follows:

α=α₀ +∫{dot over (α)}dt;  (eq. 21)

I=I ₀ +∫İdt; and  (eq. 22)

A=A ₀ +∫{dot over (A)}dt,  (eq. 23)

where α₀, I₀ and A₀ are the initial values of tool face, inclination andazimuth (e.g., approximate values derived based on a relatively coarsegyro-compassing procedure available at high latitude, or in the presenceof platform rotational motion), and

$\begin{matrix}{{\overset{.}{\alpha} = {G_{z} + {\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)\cot \; I} - \frac{\Omega_{H}\cos \; A}{\sin \; I}}};} & \left( {{eq}.\mspace{14mu} 24} \right) \\{{\overset{.}{I} = {{{- G_{x}}\cos \; \alpha} + {G_{y}\sin \; \alpha} + {\Omega_{H}\sin \; A}}};{and}} & \left( {{eq}.\mspace{14mu} 25} \right) \\{{\overset{.}{A} = {{- \frac{\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)}{\sin \; I}} + {\Omega_{\eta}\cos \; {A\cot}\; I} - \Omega_{V}}},} & \left( {{eq}.\mspace{14mu} 26} \right)\end{matrix}$

where G_(x), G_(y) and G_(z) are measurements of angular rate about thex, y and z axes of the survey tool.

System Error Equations

System error equations may be expressed as follows:

$\begin{matrix}{{{\Delta \overset{.}{\alpha}} = {{\left( {{G_{x}\cos \; \alpha} - {G_{y}\sin \; \alpha}} \right)\cos \; {I \cdot \Delta}\; \alpha} - {{\frac{\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)}{\sin^{2}I} \cdot \Delta}\; I} + {\frac{\Omega_{u}\cos \; {A\cot}\; I}{\sin \; I}\Delta \; I} + {\frac{\Omega_{H}\sin \; A}{\sin \; I}\Delta \; A} + {\sin \; \alpha \; \cot \; {I \cdot \Delta}\; G_{x}} + {\cos \; \alpha \; \cot \; {I \cdot \Delta}\; G_{y}} + {\Delta \; G_{z}}}};} & \left( {{eq}.\mspace{14mu} 27} \right) \\{{{{{\Delta \; \overset{.}{I}} = {{\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right) \cdot {\Delta\alpha}} + {\Omega_{H}\cos \; {A \cdot \Delta}\; A} - {\cos \; {\alpha \cdot \Delta}\; G_{x}} + {\sin \; {\alpha \cdot \Delta}\; G_{y}}}};}\mspace{79mu} {and}}\mspace{175mu}} & \left( {{eq}.\mspace{14mu} 28} \right) \\{{{\Delta \; \overset{.}{A}} = {{{{- \frac{\left( {{G_{x}\cos \; \alpha} - {G_{y}\sin \; \alpha}} \right)}{\sin \; I}} \cdot \Delta}\; \alpha} + {{\frac{\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)\cos \; I}{\sin \; I} \cdot \Delta}\; I} - {{\frac{\Omega_{H}\cos \; A}{\sin^{2}I} \cdot \Delta}\; I}}};{{{- \Omega_{H}}\sin \; A\; \cot \; {I \cdot \Delta}\; A} - {{\frac{\sin \; \alpha}{\sin \; I} \cdot \Delta}\; G_{x}} - {{\frac{\cos \; \alpha}{\sin \; I} \cdot \Delta}\; G_{y}}}} & \left( {{eq}.\mspace{14mu} 29} \right)\end{matrix}$

The system error equations may further be expressed in matrix form as:

{dot over (x)}=F·x+G·w,  (eq. 30)

where x=[ΔαΔIΔAΔG _(x) ΔG _(y) ΔG _(z)]^(T)  (eq. 31)

and represents the system error states, w is a 3 element vectorrepresenting the gyro measurement noise, G is the system noise matrixand the error matrix F can be given by:

$\begin{matrix}{F = \begin{bmatrix}{\left( {{G_{x}\cos \; \alpha} - {G_{y}\sin \; \alpha}} \right)\cos \; I} & \frac{{- \left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)} + {\Omega_{H}\cos \; A\; \cos \; I}}{{\sin \;}^{2}I} & \frac{\Omega_{u}\sin \; A}{\sin \; I} & {\sin \; \alpha \; \cot \; I} & {\cos \; \alpha \; \cos \; I} & 1 \\\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right) & 0 & {\Omega_{H}\cos \; A} & {{- \cos}\; \alpha} & {\sin \; \alpha} & 0 \\{- \frac{\left( {{G_{x}\cos \; \alpha} - {G_{y}\sin \; \alpha}} \right)}{\sin \; I}} & \frac{{\left( {{G_{x}\sin \; \alpha} + {G_{y}\cos \; \alpha}} \right)\cos \; I} - {\Omega_{H}\cos \; A}}{\sin^{2}I} & {{- Ι_{H}}\sin \; A\; \cot \; I} & {- \frac{\sin \; \alpha}{\sin \; I}} & {- \frac{\cos \; \alpha}{\sin \; I}} & 0 \\0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0\end{bmatrix}} & \left( {{eq}.\mspace{14mu} 22} \right)\end{matrix}$

Filter Measurement Equations

Three accelerometers in the survey system (e.g., the survey tool 30) canprovide independent measurement of tool face and inclination angles, asshown by the following equations:

$\begin{matrix}{{\overset{\sim}{\alpha} = {\arctan \left( \frac{A_{x}}{A_{y}} \right)}};{and}} & \left( {{eq}.\mspace{14mu} 33} \right) \\{{\overset{\sim}{I} = {\arctan\left( \frac{\sqrt{A_{x}^{2} + A_{y}^{2}}}{A_{z}} \right)}},} & \left( {{eq}.\mspace{14mu} 34} \right)\end{matrix}$

and it can be assumed for the purposes of this example filterformulation that an estimate of survey tool 30 azimuth (Ã) is providedby the satellite attitude determination process (e.g., using thedirectional reference system 16).

The differences between the two estimates of tool-face, inclination andazimuth can form the measurement difference inputs (z) to a Kalmanfilter, as follows:

$\begin{matrix}{z = {\begin{bmatrix}{\overset{\sim}{\alpha} - \alpha} \\{\overset{\sim}{I} - I} \\{\overset{\sim}{A} - A}\end{bmatrix}.}} & \left( {{eq}.\mspace{14mu} 35} \right)\end{matrix}$

The measurement differences (z) may also be expressed in terms of theerror states (x) as follows:

$\begin{matrix}{{z = {{H \cdot x} + {I \cdot v}}},} & \left( {{eq}.\mspace{14mu} 36} \right) \\{{where}{{H = \begin{bmatrix}1 & 0 & 0 & 0 & 0 & 0 \\0 & 1 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 0 & 0 & 0\end{bmatrix}},}} & \left( {{eq}.\mspace{14mu} 37} \right)\end{matrix}$

v may be a 3 element vector that represents the accelerometermeasurement and GPS azimuth measurement noise, and I is a measurementnoise matrix.

Kalman Filter Equations

Discrete System and Measurement Equations

While the system may be described mathematically in the continuousdifferential equation form given above, the measurements are in practiceprovided at discrete intervals of time. To address with this, and toprovide a computationally efficient filtering algorithm, the continuousequations can be expressed in the form of difference equations as shownbelow:

x _(k+1)=Φ_(k) ·x _(k)+Δ_(k) ·w _(k);  (eq. 38)

where Φ_(k)=exp [F·(t _(k+1) −t _(k))],  (eq. 39)

with measurements expressed as:

z _(k+1) =H _(k+1) ·x _(k+1) +v _(k+1),  (eq. 40)

and where

-   -   x_(k)=error state at time t_(k),    -   w_(k)=system noise at time t_(k),    -   Φ_(k)=state transition matrix from time t_(k) to time t_(k+1),    -   Δ_(k)=system noise matrix at time t_(k),    -   z_(k+1)=measurement difference at time t_(k+1),    -   v_(k+1)=measurement noise at time t_(k+1), and    -   H_(k+1) measurement matrix calculated at time t_(k)+₁.

The noise can be zero mean, but now discrete, and can be characterizedby the covariance matrices Q_(k) and R_(k) respectively.

Prediction Step

A relatively accurate estimate (e.g., a best estimate) of the errorstate at time t_(k) is denoted below by x_(k/k). Since the system noisew_(k) of certain embodiments has zero mean, the best prediction of thestate at time t_(k+1) can be expressed as:

x _(k+1/k)=Φ_(k) ·x _(k/k)  (eq. 41)

while the expected value of the covariance at time t_(k+1) predicted attime t_(k), can be given by:

P _(k+1/k)=Φ_(k) ·P _(k/k)·Φ_(k) ^(T)+Δ_(k) ·Q _(k)·Δ_(k) ^(T)  (eq. 42)

Measurement Update

The arrival of a new set of measurements z_(k+1) at time t_(k+1) can beused to update the prediction to generate a relatively accurate estimate(e.g., a best estimate) of the state at this time. For example, arelatively accurate (e.g., best) estimate of the state at time t_(k+1)can be expressed as:

x _(k+1/k+1) =x _(k+1/k) −K _(k+1) [H _(k+1) x _(k+1/k) −z_(k+1)],  (eq. 43)

and its covariance by:

P _(k+1/k+1) =P _(k+1/k) −K _(k+1) H _(k+1) P _(k+1/k),  (eq. 44)

where the Kalman gain matrix can be given by:

K _(k+1) =P _(k+1/k) H _(k+1) ^(T) [H _(k+1) P _(k+1/k) H _(k+1) ^(T) +R_(k+1)]⁻¹.  (eq. 45)

State Correction

Following each measurement update, the states can be corrected usingcurrent estimates (e.g., best estimates) of the errors. In thissituation, the predicted state errors become zero:

x _(k+1/k)=0.  (eq. 46)

F. Initialization of the Survey Tool on a Moving Surface

In certain circumstances, the apparatus 10 may be positioned on a movingsurface. For example, the apparatus 10 may be on an off-shore drillingrig or platform. The continuous survey mode will generally operateproperly on the Earth under such conditions, provided some means ofinitializing the integration process involved, other thangyro-compassing, can be established. For example, given some independentmeans of keeping track of the substantially instantaneous attitude of amoving platform, and the dynamic transfer of that information to thesurvey tool to initialize the continuous survey process, the potentialexists to remove the survey uncertainties associated with platformmotion. It can therefore be beneficial to maintain a dynamic orientation(e.g., reference attitude) on the moving surface (e.g., a rig) which canbe initialized at a particular moment. For example, the orientation(e.g., reference attitude or azimuth) of the survey tool 30 with respectto the reference direction 18 can be determined and/or transferred tothe survey tool 30 generally immediately before the tool is placed incontinuous survey mode (e.g., upon insertion of the survey tool 30 intothe wellbore) in accordance with certain embodiments. In certainembodiments, the directional reference system 16 and/or the inertialnavigation system 42 may be used to conduct the determination, transferthe information regarding the orientation to the survey tool 30, orboth, as described herein (e.g., with respect to FIG. 6).

In some other embodiments, the motion of the drilling rig or platformmay be advantageously used to initialize the survey tool 30. Forexample, an angular rate measurement matching procedure may be used todetermine the relative orientation (e.g., attitude and/or azimuth)between two orthogonal sets of axes on the platform structure (e.g.,between a set of axes defined by the inertial navigation system 42 and aset of axes defined by the survey tool 30). Such a procedure may accountfor relative differences between the orientation of the survey tool 30and the apparatus 10. In general, as described herein, initialization ofthe survey tool 30 using the apparatus 10 can be achieved accuratelywhere the wellbore survey tool 30 is mounted in some predeterminedorientation with respect to the apparatus 10 or components thereof(e.g., the directional reference system 16). Thus, the accuracy of thedetermination of the orientation of the survey tool 30 may be improvedwhen the alignment of the survey tool 30 (e.g., attitude) with respectto the apparatus 10 is relatively accurate and/or precise. Using theangular rate matching process described herein, residual misalignmentsbetween the survey tool 30 and the apparatus 10 may be determined suchthat actual mounting alignment accuracy of the survey tool 30 on theapparatus 10 becomes less critical.

Examples of a generally similar angular rate matching procedure used toproduce precision alignment in attitude and corresponding systems foraligning a weapons system on a sea-borne vessel are described in U.S.Pat. No. 3,803,387, entitled “Alignment Error Detection System,” whichis hereby incorporated in its entirety by reference herein. By comparingthe sets of angular rate measurements (e.g., from the inertialnavigation system 42 and the survey tool 30), it is possible to deducethe relative orientation of the two sets of axes (e.g., o the apparatus10 and the survey tool 30). The orientation of the apparatus 10 (whichmay be referred to as the platform reference frame) may be defined bythe orientation of the inertial navigation system 42, an integrateddevice 43 (e.g., an integrated GPS/AHRS unit), or the directionalreference system 16.

In an offshore drilling or platform, for example, the rocking motion ofthe rig is generally sufficient to provide angular motion sufficient toallow the attitude determination. Accurate knowledge of the inertialnavigation system 42 reference orientation with respect to thegeographic reference frame (e.g., the reference direction 18), combinedwith knowledge of the relative orientation (e.g., attitude and/orazimuth) between the survey tool 30 and the inertial navigation system42 according to an angular rate matching procedure, can allow foraccurate determination of the orientation (e.g., attitude and/orazimuth) of the survey tool 30 with respect to the geographic referenceframe (e.g., the reference direction 18). Advantageously, utilizing theangular rate matching procedure, the initial orientation of the surveytool 30 can be accurately obtained in situations where the tool 30 isphysically misaligned with respect to the platform reference system(e.g., due to operator error in mounting the tool, misalignment due toimprecision in the manufacturing/assembly of the platform, etc.). Incertain embodiments, the directional reference system 16, or anintegrated unit comprising a directional reference system 16 and aninertial navigation system 42 (e.g., GPS/INS unit 43), is used insteadof or in addition to the inertial navigation system 42 in the angularrate matching procedure.

FIG. 10 is a flowchart of an example method 300 of initializing awellbore survey tool 30 utilizing an angular rate matching procedure.While the method 300 is described herein by reference to the apparatus10 described with respect to FIGS. 2-8, other apparatus described hereincan also be used (e.g., the apparatus 400 of FIG. 10). At operationalblock 302, the method 300 comprises receiving a first signal indicativeof an orientation of a directional reference system 16 with respect to areference direction 18. For example, the orientation of the directionalreference system 16 may be calculated by a processor of the directionalreference system 16 in response to signals received by the first antenna22 and the second antenna 24 as described herein. The first signal maybe generated by the directional reference system 16 and transmitted forprocessing (e.g., to the computing system 52 or directly to the wellboresurvey tool 30). In certain embodiments, the method 300 furthercomprises positioning the wellbore survey tool 30 such that the wellboresurvey tool 30 has a predetermined orientation with respect to thedirectional reference system 16. For example, the wellbore survey tool30 may be positioned substantially parallel with the directionalreference system 16 on the apparatus 10 (e.g., using a tool positioningelement as described herein).

The method 300 further comprises receiving a second signal indicative ofthe rate of angular motion of the directional reference system 16 atoperational block 304. For example, in certain embodiments, one or moresensors (e.g., one or more gyroscopes) of the inertial navigation system42 measure the rate of angular motion of the inertial navigation system42 and generate the second signal indicative of the same. The inertialnavigation system 42 may then transmit the second signal for processing(e.g., to the computing system 52 or directly to the wellbore surveytool 30). In certain other embodiments, the rate of angular motion ismeasured directly by the directional reference system 16. In oneembodiment, apparatus 10 comprises an integrated system, such as theintegrated GPS/AHRS unit 43. In such an embodiment, because thedirectional reference system 16 is integrated with the inertialnavigation system 42, the GPS/AHRS unit 43 generates the second signal.

At operational block 306, the method 300 comprises receiving a thirdsignal indicative of the rate of angular motion of a wellbore surveytool 30. For example, one or more sensors of the survey tool 30 (e.g.,one or more gyroscopes) may measure the rate of angular motion of thesurvey tool 30 and generate the third signal. The third signal may thenbe transmitted for processing (e.g., to the computing system 52 ordirectly to the wellbore survey tool 30).

The method 300 can further comprise determining a relative orientationof the directional reference system 16 and the wellbore survey tool 30in response to the second signal and the third signal at operationalblock 308. For example, the relative orientation can be determined usingan angular rate matching procedure described herein. At operationalblock 310, the method 300 of certain embodiments comprises determiningan orientation of the wellbore survey tool 30 with respect to thereference direction 18 in response to the first signal and the relativeorientation. Given the orientation of the directional reference system16 with respect to the reference direction 18, as indicated by the firstsignal, and given the relative orientation of the survey tool 30 to thedirectional reference system 16, as indicated by the angular ratematching procedure, such a determination can be made.

In certain embodiments, the second signal may be indicative of the rateof angular motion of the inertial navigation system 42, or of generallythe entire apparatus 10 or components thereof (e.g., the base portion12), instead of, or in addition to the directional reference system 16.For example, in one embodiment, the second signal is generated by theinertial navigation system 42 and is directly indicative of theorientation of the inertial navigation system 42 with respect to thereference direction 18. For example, the inertial navigation system 42may be oriented in substantially the same orientation on the apparatus10 with respect to the survey tool 30 as the directional navigationsystem 16 is oriented with respect to the survey tool 30 and istherefore at least indirectly indicative of the orientation of thedirectional reference system 16 with respect to the reference direction18.

G. Example Angular Rate Matching Filter for the Transfer of OrientationData (e.g., Attitude and Heading Reference Data) to the Survey Tool on aMoving Platform

As described, in some embodiments, the apparatus 10 includes anintegrated unit, such as a GPS/AHRS reference system 43 generallyincluding the functionality of both a directional reference system 16and an inertial navigation system 42. On a moving apparatus 10 (e.g., amoving platform or board), the azimuth difference between the surveytool 30) GPS/AHRS reference system 43 and the survey tool 30 may bedetermined by comparing angular rate measurements provided by the twosystems, provided that the drilling rig exhibits some rocking motion.For example, the measurements may be processed using a Kalman filterbased on an error model of an inertial system in the survey tool 30. Oneform of the measurement equation is expressed below. In certain otherembodiments, as described herein, separate directional reference system16 and inertial navigation system 42 are used. Such embodiments are alsocompatible with the example described herein. For example, in oneembodiment, the directional reference system 16 and the inertialnavigation system 42 comprise separate units but are substantiallyaligned with respect to each other on the apparatus 10.

The measurements of turn rate provided by the GPS/AHRS reference system43 and survey tool 30 system can be assumed to be generated in localco-ordinate frames denoted a and b respectively. In certain embodiments,the rates sensed by a triad of strap-down gyroscopes mounted at eachlocation with their sensitive axes aligned with these reference framesmay be expressed as ω^(a) and ω^(b). The measurements provided by thegyroscopes in the reference and aligning systems are resolved into acommon reference frame, the a-frame for example, before comparison takesplace.

Hence, the reference measurements may be expressed as:

z=ω ^(a),  (eq. 47)

assuming the errors in the measurements are negligible. The estimates ofthese measurements generated by the survey tool 30 system are denoted bythe ̂ notation.

{circumflex over (z)}=Ĉ _(b) ^(a){circumflex over (ω)}^(b).  (eq. 48)

The gyroscope outputs ({circumflex over (ω)}^(b)) may be written as thesum of the true rate (ω^(b)) and the error in the measurement (δω^(b))while the estimated direction cosine matrix may be expressed as theproduct of a skew symmetric error matrix, [I−φx], and the true matrixC_(b) ^(a) as follows:

{circumflex over (z)}=[I−φx]C _(b) ^(a)└ω^(b)+δω^(b)┘.  (eq. 49)

Expanding the right hand side of this equation and ignoring errorproduct terms gives:

{circumflex over (z)}=C _(b) ^(a)ω^(b) −φ×C _(b) ^(a)ω^(b) +C _(b)^(a)δω^(b).  (eq. 50)

The measurement differences may then be written as:

$\begin{matrix}\begin{matrix}{{\delta \; z} = {z - \hat{z}}} \\{= {{{- \left\lbrack {C_{b}^{a}\omega^{b}} \right\rbrack} \times \phi} - {C_{b}^{a}{\delta\omega}^{b}}}}\end{matrix} & \left( {{eq}.\mspace{14mu} 51} \right)\end{matrix}$

The measurement differences (δz_(k)) at time t_(k) may be expressed interms of the error states (δx_(k)) as follows:

δz _(k) =H _(k) δx _(k) +v _(k),  (eq. 52)

where H_(k) is the Kalman filter measurement matrix which can beexpressed as follows:

$\begin{matrix}{{H_{k} = \begin{bmatrix}0 & \omega_{z} & {- \omega_{y}} & c_{11} & c_{12} & c_{13} \\{- \omega_{z}} & 0 & \omega_{x} & c_{21} & c_{22} & c_{23} \\\omega_{y} & {- \omega_{x}} & 0 & c_{31} & c_{32} & c_{33}\end{bmatrix}},} & \left( {{eq}.\mspace{14mu} 53} \right)\end{matrix}$

where ω_(x), ω_(y), and ω_(z) are the components of the vector C_(b)^(a)ω^(b), c₁₁, c₁₂, . . . etc. are the elements of direction cosinematrix C_(b) ^(a) and v_(k) is the measurement noise vector. Thisrepresents the noise on the measurements and model-mismatch introducedthrough any flexure of the platform structure that may be present.

A Kalman filter may be constructed using the measurement equation and asystem equation of the form described above in relation to the attitudematching filter. The filter provides estimates of the relativeorientation of the platform reference (e.g., the GPS/AHRS referencesystem 43) and the survey tool 30.

H. Alternative Embodiments

FIG. 11 schematically illustrates an example apparatus 400 for moving awellbore survey tool. The apparatus 400 of FIG. 11 is configured totransport the survey tool 30 along a surface beneath the apparatus 400.In certain embodiments, the apparatus 400 is configured to bemechanically coupled to at least one directional reference system 416(e.g., on the apparatus 400 itself or on a platform configured to beremovably coupled to the apparatus 400). In this way, certainembodiments advantageously decouple the transportation functionalityfrom the orientation-determination functionality.

The apparatus 400 of certain embodiments comprises at least one support402 and a base portion 403 mechanically coupled to the at least onesupport 402. The apparatus 400 can further comprise a tool receivingportion 404 mechanically coupled to the base portion 403 and configuredto receive a wellbore survey tool 406. The apparatus 400 may alsocomprise at least one member movably coupled to a portion of theapparatus 400 and configured to allow the apparatus to move along asurface beneath the apparatus 400. The apparatus 400 can furthercomprise a tool positioning element 408 configured to controllably movethe wellbore survey tool 406 between a first position relative to theapparatus and a second position relative to the apparatus 400.

As shown in FIG. 11, the base portion 403 may comprise a substantiallyrigid, generally rectangular platform structure including a generallyplanar surface 405. In other embodiments, the base portion 12 may have adifferent shape (e.g., circular, ovular, trapezoidal, etc.), may besomewhat flexible, and/or may include one or more inclined surfaces,declined surfaces, stepped portions, etc. The base portion 403 may besimilar to the base portion 12 of the apparatus 10 described above(e.g., with respect to FIG. 2 and FIG. 4), for example.

The at least one support 402 may comprise one or more posts. Theapparatus 400 of FIG. 11 comprises three supports 402. In otherembodiments, there may be more or less supports 402 and/or the supports402 may be shaped differently (e.g., as rectangular posts, blocks,hemispherical protrusions, etc.). In various embodiments, the at leastone support may be similar to the at least one leveler 48 of theapparatus 10 described above (e.g., with respect to FIG. 4).

The tool receiving portion 404 of certain embodiments comprises an areaof the base portion 403 on which the well survey tool 406 is mounted. Invarious embodiments, the survey tool 406 can be releasably secured tothe tool receiving portion 404. In certain embodiments, the toolreceiving portion 403 is similar to the second mounting portion 20 ofthe apparatus 10 described above (e.g., with respect to FIG. 2).

The surface beneath the apparatus 400 may be the Earth's surface, a rigsurface, etc. In certain embodiments, the at least one member comprisesa wheel, tread, ski, or other mechanism configured to allow for movementof the apparatus 400 along the surface. In some embodiments, forexample, the at least one member of the apparatus 400 is similar to theat least one member of the apparatus 10 described above (e.g., withrespect to FIG. 4).

The tool positioning element 408 can be configured to controllably movethe wellbore survey tool 406 between a first position relative to theapparatus 400 and a second position relative to the apparatus 400. Incertain embodiments, the first position is horizontal with respect tothe base portion 403 and the second position is vertical with respect tothe base portion 403. The tool positioning element 408 may be similar tothe tool positioning element 56 of the apparatus 10 described above(e.g., with respect to FIGS. 6A-6C) in certain embodiments.

The apparatus 400 may further comprise a mounting portion 414mechanically coupled to the base portion 403 and configured to receiveat least one directional reference system 416. The at least onedirectional reference system 416 can be configured to provide data(e.g., attitude or azimuth) indicative of an orientation of the at leastone directional reference system 416 with respect to a referencedirection. In certain embodiments, the mounting portion 414 is similarto the first mounting portion 14 of the apparatus 10 described above(e.g., with respect to FIG. 2).

The directional reference system 416 may be similar to the directionalreference system 16 described above (e.g., with respect to FIG. 2). Forexample, the at least one directional reference system 416 comprises atleast one signal receiver of a global positioning system (GPS). Forexample, the directional reference system 16 may comprise a firstantenna 418 and a second antenna 420 spaced apart from the first antennaand defining a line 422 from the first antenna 418 to the second antenna420. In certain embodiments, the at least one signal receiver furthercomprises a processor (not shown) configured to receive signals from thefirst and second antennae 418, 420 and to determine an orientation ofthe line 422 (e.g., attitude or azimuth) with respect to the referencedirection 424.

In certain embodiments, the tool receiving portion 408 is configured toreceive the wellbore survey tool 406 such that the wellbore survey tool406 has a predetermined orientation with respect to the at least onedirectional reference system 416. This general configuration may besimilar the one described above (e.g., with respect to FIG. 2) for theapparatus 10, the wellbore survey tool 30, and the directional referencesystem 16, for example. In addition, the survey tool 406 of certainembodiments may be similar to the survey tool 30 described above (e.g.,with respect to FIG. 2).

The apparatus 400 of certain embodiments may further include one or moreof components described herein, such as an inertial navigation systemand/or computing system similar to the inertial navigation system 42 andcomputing system 52 of the apparatus 10 described above (e.g., withrespect to FIG. 4).

I. Remote Reference Source

Certain embodiments described above include methods and apparatus forinitializing a wellbore survey system using an external directionalreference system such as a satellite navigation system (GPS/GLONASS).One of the methods described generally involves mounting both thesatellite reference system (e.g., comprising 2 or more antennae,receivers and processor) and the survey tool on a stable platform in aknown orientation with respect to one another and transferring attitudedata from the reference system to the tool. Thereafter, the tool isswitched to a continuous survey mode allowing its orientation to betracked during pick-up of the tool and positioning at the entrance tothe well, and throughout the subsequent survey of the well.

In certain cases, screening of the GPS antennae may occur (e.g., by thederrick or other objects). Thus, it can be advantageous to mount the GPSwell away from the derrick and so have a sufficient number of satellitesin view. However, it can also be desirable to mount the survey tool inclose proximity to the well head/Kelly bushing (e.g., near to theentrance to the wellbore) so as to avoid having to transport the tool tothis location after initialization. Survey errors can propagatethroughout the period of tool surface handling—therefore it is oftendesirable to keep this to a minimum duration. Further, there is apossibility of exceeding the dynamic range of the sensors in the tool,e.g. of saturating the gyroscopes by exceeding maximum allowable inputrate. If this occurs, the attitude reference stored in the tool atinitialization will be lost and the procedure of aligning the tool tothe GPS reference will need to be repeated. Thus, there can be a tensionbetween these two design goals: performing initialization using GPSmeasurements on the rig and positioning the tool close to the wellhead/Kelly bushing tool to minimize the surface handling requirement.

To address the competing design goals described above, certain methodsdescribed herein involve mounting the GPS equipment and the survey toolremote from one another during the initialization process. For example,the GPS equipment can be mounted well away from the derrick (e.g., inorder to maximize the number of satellites in view) and the tool may belocated close to the entrance to the well (e.g., in order to minimize orotherwise reduce the movement of the tool prior to running into the welland/or the time taken in any physical transfer of the tool between twolocations). In certain embodiments, the initial orientation of thewellbore survey tool is determined with respect to a chosen referenceframe (e.g., the local vertical geographic frame expressed as an azimuthangle, an inclination, and a high-side orientation of the wellboresurvey tool). In certain embodiments described herein, the directionalreference system and the wellbore survey tool are not mechanicallycoupled to one another and are mounted on respective surfaces that arenot mechanically coupled to one another.

FIG. 12 is a flowchart of an example method 500 for determining anorientation of a wellbore survey tool at a first position with respectto a reference direction in accordance with certain embodimentsdescribed herein. In an operational block 510, the method 500 comprisesreceiving information (e.g., at least one first signal) indicative of anorientation of a directional reference system with respect to thereference direction. The directional reference system is positioned at asecond position spaced from the first position. In an operational block512, the method 500 further comprises receiving information (e.g., atleast one second signal) indicative of a relative orientation of thewellbore survey tool with respect to the directional reference system.In an operational block 514, the method 500 further comprisesdetermining the orientation of the wellbore survey tool at the firstposition in response at least in part to the received information (e.g.,the at least one first signal and the at least one second signal).

In certain embodiments, the at least one first signal and the at leastone second signal are received by a computer system comprising one ormore computer processors (e.g., one or more computer microprocessors).For example, the one or more computer processors can comprise one ormore processors of the wellbore survey tool, the directional referencesystem, or one or more processors that are dedicated to determining theorientation of the wellbore survey tool. Additional information, such asparameter values (e.g., distance between two reference points on thewellbore survey tool, distance between two reference points on thedirectional reference system, distance between the wellbore survey tooland the directional reference system, and horizontal and verticalcomponents of these distances) that are directly or indirectlyrepresentative of one or more dimensions or geometric relationships ofor between the wellbore survey tool and the directional reference system(e.g., angle between lines linking reference points and axes of tool andGPS reference directions) may also be used in determining theorientation of the wellbore survey tool, and such parameter values arereceived by the one or more processors which are used to calculate theorientation of the wellbore survey tool. In certain embodiments, the oneor more computer processors comprise one or more inputs to receive data(e.g., information or one or more signals) indicative of (e.g., to beused to compute) the orientation of the directional reference systemwith respect to the reference direction and indicative of the relativeorientation of the wellbore survey tool with respect to the directionalreference system.

In certain embodiments, the computer system further comprises a memorysubsystem adapted to store information (e.g., one or more signals orparameter values) to be used in the determination of the orientation ofthe wellbore survey tool. The computer system can comprise hardware,software, or a combination of both hardware and software. In certainembodiments, the computer system comprises a standard personal computer.In certain embodiments, the computer system comprises appropriateinterfaces (e.g., modems) to receive and transmit signals as needed. Thecomputer system can comprise standard communication components (e.g.,keyboard, mouse, toggle switches) for receiving user input, and cancomprise standard communication components (e.g., image display screen,alphanumeric meters, printers) for displaying and/or recording operationparameters, orientation and/or location coordinates, or otherinformation used in determining the orientation or generated as a resultof determining the orientation. In certain embodiments, the computersystem is configured to read a computer-readable medium (e.g., read-onlymemory, dynamic random-access memory, flash memory, hard disk drive,compact disk, digital video disk) which has instructions stored thereonwhich cause the computer system to perform a method for determining anorientation of the wellbore survey tool in accordance with certainembodiments described herein. In certain embodiments, at least onesignal of the at least one first signal and the at least one secondsignal is received from user input, computer memory, or sensors or othercomponents of the system configured to provide signals having thedesired information.

Techniques are also described herein for transferring the attitudereference defined by the GPS to a location physically removed from it(e.g., the tool location). In certain embodiments, the wellbore surveytool is at a first position spaced a first distance from the wellboreentrance (e.g., spaced a first distance from the well head/Kellybushing) and the directional reference system is at a second positionspaced a second distance from the wellbore entrance (e.g., spaces asecond distance from the well head/Kelly bushing), with the seconddistance being greater than the first distance. In certain embodiments,the first distance has a first horizontal component that is less than 10feet, or the second distance has a second horizontal component that isgreater than the first horizontal component by at least about 30 feet,or both. In certain embodiments, the first distance has a first verticalcomponent that is less than about 20 feet.

In some cases, the horizontal separation distance between the firstposition and the second position could be as much as 50 feet, and thetwo positions could be at different levels on the rig (also up to 50feet). In other configurations, the horizontal and vertical separationdistances can vary. For example, in various configurations, thehorizontal and/or vertical separation distances may range from betweenabout 10 and 1000 feet, may be at least 10, 15, 20, 25, 30, 35, 40, 45,50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 1000 feet, or may be somevalue greater than 1000 feet. For example, in certain such embodiments,the GPS equipment (or other directional reference system) and the surveytool are separated by a distance beyond a distance for which it isphysically easy or straightforward to have the GPS equipment and thesurvey tool mechanically connected to one another. Moreover, in somecases, the survey tool and the GPS equipment are mounted during theinitialization process such that they are not mechanically coupled toone another, are mounted on respective surfaces that are notmechanically coupled to one another, or both.

In certain embodiments, the information (e.g., the at least one firstsignal) indicative of an orientation of the directional reference systemwith respect to the reference direction is generated or provided by thedirectional reference system itself. For example, the directionalreference system can generate one or more signals based on theorientation of the directional reference system, and can input the oneor more signals to the one or more computer processors.

Furthermore, a number of methods are described herein generate theinformation (e.g., the at least one second signal) indicative of therelative orientation of the wellbore survey tool with respect to thedirectional reference system e.g., using either (i) laser/opticalsighting between the GPS reference equipment and the tool or (ii) theapplication of an inertial attitude reference system. In both cases, thesurvey tool may be mounted vertically, horizontally, or anywhere inbetween during the attitude initialization process. Provided the toolcan be physically located close to the entrance to the well at thistime, any need to move the tool over a significant distance followingGPS attitude initialization is avoided or reduced and the time forattitude errors to propagate before the start of a wellbore survey istherefore reduced. If the tool can be held close to vertical during thisprocess, the need to rotate the tool before insertion in the well isalso avoided or reduced. Therefore, by holding the survey tool verticalclose to the wellbore entrance (e.g., the well head/Kelly bushing)throughout the initialization process, attitude errors which would growand contribute to the overall attitude error at the start of a surveymay be kept to a minimum or are otherwise significantly reduced.Techniques are described here which address these issues.

It is desirable to accurately determine, the full attitude of the surveytool, e.g., the azimuth, inclination and high side orientation withrespect to the chosen reference frame (the local vertical geographicframe for example). It is therefore desirable for the attitude referenceto be capable of defining fully the attitude of the tool forinitialization purposes, particularly for operation on a moving offshoreplatform. It is noted that whilst the inclination and high side anglescan be determined very accurately on a stationary platform using themeasurements provided by the accelerometers installed in the tool, thisapproach is less reliable offshore, and may not produce accurateresults.

However, for the purposes of illustrating and providing a clear (flatpage) visualization of the techniques described below, single planeillustrations are given, and attention is focused on the determinationof tool orientation with respect to true north which is used as the toolazimuth angle. In the event that the tool is mounted at, or close to,the local vertical, it is desirable to determine the direction of alateral axis of the tool (usually the y-axis) with respect to north. Thedirection of the projection of this lateral axis on the horizontalplane, with respect to north, is commonly referred to as the gyro toolface angle.

It is stressed that some or all of the methods described herein may beadapted and used to define the attitude of the survey tool completely,and made to work irrespective of the orientation of the survey tool. Insuch cases, the system geometry will become more complex and additionalmeasurements may be taken and used to extract full attitude data.

1. Optical Sighting Procedures

In certain embodiments, one or more optical sighting procedures are usedto generate information (e.g., the at least one second signal)indicative of the relative orientation of the wellbore survey tool 530with respect to the directional reference system 540. FIG. 13illustrates an example wellbore survey tool/directional reference systemarrangement and corresponding initialization process that may beimplemented when the survey tool 530 is horizontal. A theodolite and aranging device (not shown) mounted on the platform containing thesatellite antennae provides measurements of the line of sight to twopoints marked at a known spacing along the tool casing. Together withmeasurements of the ranges to each of these points, it is possible todefine fully the triangle formed by the location of the theodolite andtwo known points on the casing of the tool 530. Given this information,the direction in which the tool is pointing with respect to north (thetool azimuth) may be calculated using the geometric relationships shownin FIG. 13. For example, the reference azimuth (A_(R)) can be determinedusing the directional reference system 540 (e.g., satellite referencesystem), and angles θ₁ and θ₂ and distances R₁ and R₂ can be measured.Angles α and β can be computed, which are functions of measureddistances R₁ and R₂ and the difference Δθ between angles θ₁ and θ₂. Thetool azimuth can then be computed using A_(T)=A_(R)−θ₁−α+180 orA_(T)=A_(R)−θ₂+β.

The accuracy of the process described may be limited by the ability tosite on to the appropriate points on the survey tool casing, but may beenhanced by taking multiple measurements at known spacing along thecasing. By this method some redundancy is introduced into themeasurement data, and the measurements may then be processed using aleast squares adjustment.

Whilst the procedure and calculation described in FIG. 13 is valid forthe situation where the tool is horizontal, the method can be extendedto cases in which the tool is mounted in any orientation with respect tothe reference frame. In such cases, both the geometrical arrangement andthe calculations used to determine the orientation of the tool becomemore complex, but are within the capability of persons of ordinary skillin the art using the disclosure herein.

If the tool were to be mounted vertically, a similar process may beimplemented. For example, the orientation of a mirror 532 attached tothe tool 530 aligned perpendicular to a known axis (e.g., the y-axis asdepicted in FIG. 14) may be determined. The angle measured with respectto a reference direction and the angle of the reference direction withrespect to north may then be summed to determine the gyro tool faceangle. According to this approach, it is desirable to accurately alignand position the mirror 532 with respect to the axes of the survey tool530. A method of achieving this alignment is described below.

The tool 530 can be mounted horizontally in a v-shaped channel or blockmount(s) 550 and a flat bar 552 can be positioned above the tool 530 asshown in FIG. 15. The bar 552 may be leveled accurately using a levelsensor 554 attached to the bar 552. A laser 556 can be positioned on thebar 552 with its beam pointing perpendicular to it, e.g., alignedvertically. Using x and y accelerometer measurements, the tool high sideangle can be determined, which corresponds to the angle between they-axis of the tool 530 and the laser beam direction. For example, thetool high side angle α can be expressed using the x accelerometermeasurement (A_(X)) and the y accelerometer measurement (A_(y)) asα=tan⁻¹(A_(x)/A_(y)). If the tool 530 is subsequently lifted to thevertical and the direction of the laser beam with respect to true northcan be established, the gyro tool face angle can be determined by simplysumming the high side angle, measured when the tool 530 was horizontal,and the beam angle. Thus, in certain embodiments, the tool highsideangle is determined while the wellbore survey tool 530 is substantiallyhorizontal (e.g., aligned with the local horizontal using the levelsensor), and the wellbore survey tool 530 is then moved to besubstantially vertical, and the orientation of the wellbore survey tool530 at the first position is determined by calculating the gyro toolface angle (e.g., using accelerometer measurements from the wellboresurvey tool 530) at least in part based on the determined tool highsideangle.

A similar result may be achieved by replacing the laser 556 with amirror attached to the bar 552 described above. A method of determiningthe gyro tool face angle is described next with respect to FIGS. 16-18.

According to such a method, the satellite antennae 542 of thedirectional reference system 540 are mounted on a platform as describedpreviously. Also mounted on this platform can be a laser light source544 coupled with an optical sight and a mirror 546 which can be bothrotated and moved along the axis of the platform as depicted in FIG. 16.A motor driven screw mechanism may be used to achieve linear motion ofthe mirror 546 along the reference axis 548, and a further motor can beincorporated to rotate the tool 530 to the desired angle. The laser beamcan be directed or transmitted along a first line extending between thedirectional reference system 540 and the centre of the reflectingsurface of the mirror 532 attached to the survey tool 530, or at a flatsurface machined on the casing of the tool 530. The mirror 532 or flatsurface on the casing of the tool 530 is at a predetermined orientationwith respect to the tool 530, and reflects the incident light. Incertain embodiments, the wellbore survey tool 530 at the first positionis mounted substantially vertically with respect to the wellboreentrance. In certain embodiments, the mirror 532 is moved to change thedirection the light is reflected by the mirror 532, and since the mirror532 is mechanically coupled to the wellbore survey tool 530, the mirror532 and tool 530 maintain their relationship with one another whilebeing moved.

The light reflected by the mirror 532 is transmitted along a second lineextending between the mirror 532 and a movable mirror 546 on thereference platform. The movable mirror 546 is positioned to intersectthe beam reflected from the tool mounted mirror 532 and subsequentlyrotated in order to reflect or direct the beam back along the axis 548of the reference platform. The operator or other entity makes thenecessary linear and angular adjustments to this mirror 546 to ensurethat the returning beam from the tool mounted mirror 532 is directed ata target point alongside the laser source. In certain embodiments, thelight reflected by the mirror 546 propagates along a third lineextending between the mirror 546 and a portion of the directionalreference §ystem (e.g., the light source 544), such that the first line,the second line, and the third line form a triangle.

The resulting triangle (denoted ABC) formed by the light path (A to C toB to A) is shown in FIG. 17A. The geometry of this triangle can be fullydefined using the measured angles which are shown in FIG. 17A. Point Odenotes the central axis of the survey tool 530, and the lateral axes ofthe tool Ox and Oy are also shown in FIG. 17A. Other measured angles arethe beam angle θ with respect to the azimuth reference, mirror angleρ_(m) with respect to the azimuth reference, and the tool y-axis α withrespect to the tool mirror axis (corresponding to the measured highsideangle). Given knowledge of the reference azimuth axis AB direction withrespect to north (defined by the satellite system and corresponding tothe reference azimuth angle Ψ_(o)), the internal angles of the triangleABC and the orientation of the tool axis Oy with respect to the axis ofthe mirror 532 attached to the tool 530, the orientation of the toolaxis Oy with respect to north (the gyro tool face angle) can bedetermined.

An example sequence of calculations used to establish this angle, usingthe angles shown in FIG. 17B, is now described. The azimuth referencedirection Ψ₀ is defined by the directional reference system 540, is thedirection of line AB with respect to north. The direction of line BCwith respect to north, defined by azimuth reference Ψ₀ and mirror angleρ_(m) is given by Ψ₁=Ψ₀+2ρ_(m). The direction of line CO with respect tonorth, defined by Ψ₁ and measured angle θ, is given byΨ₂=Ψ₁+180−ρ_(m)+θ/2=Ψ₀+180+ρ_(m)+θ/2. The direction of tool axis (Oy)with respect to north (gyro toolface angle), defined by Ψ₂ and measuredhighside angle α, is given by Ψ₃=Ψ₂+α−360=Ψ₀+ρ_(m)+θ/2+α−180.

Additional geometric measurements may be provided to aid the processdefined in FIG. 17B. For example, the distance between the laser sourceand the movable mirror (AB) may be measured and used in thecomputational process to determine tool orientation (shown in FIG. 17A).The availability of additional measurement data such as this may be usedto advantage to check the accuracy of the computational process andprovide quality control, through a least squares adjustment process forexample.

In alternative embodiments and as illustrated in FIG. 18, the gyro toolface angle and/or other parameters can be determined using a mirror 532attached to the tool 530 (e.g., at the highside point), and anautocollimating head 549 attached to the directional reference system540 (e.g., a GPS unit or fixture). The autocollimating head 549 and themirror 532 can then be aligned via a visual sighting, or a light beam,for example. In such an arrangement, it may be desirable that the mirror532 be locked in the “gyro tool face” plane, but able to be tilted inthe inclination plane to allow any differences in height to beaccommodated. During the autocollimation process, a beam of light can besent out through the head 549 and the reflection can be detected comingback onto the eyepiece. In other embodiments, alignment can bedetermined by detecting that the image of the end of the autocollimatinghead 549 is in the mirror reflection (e.g., when looking through theeyepiece), indicating that the mirror 532 and head 549 are lined up orsubstantially lined up with each other.

A further alternative scheme for establishing the instantaneous gyrotool face angle of a survey tool on a moving platform is described next.The following method relies on the accurate surveying of theorientations of two mounting locations on the rig, one for the satellitereference antennae and one for the survey tool, each with respect to adefined platform reference frame. Given that the survey tool is clampedin the defined reference location, and that its orientation relative tothe satellite reference system is known to an acceptable level ofaccuracy, the satellite reference can be transferred to the survey tooland the survey process initiated. In the following description, it isassumed throughout that the rig structure is substantially rigid andthat the relative orientations of the mounting locations are thereforesubstantially unchanging.

The transformations between the various coordinate frames are denoted bydirection cosine matrices, viz.

C_(G) ^(R)=coordinate transformation from the local geographic reference(G), defined by the directions of true north, east and the localvertical, and the satellite reference frame (R)—established using thesatellite system.

C_(P) ^(R)=coordinate transformation from the platform reference (P) andthe satellite reference frame (R)—determined using standard landsurveying procedures

C_(P) ^(T)=coordinate transformation from the platform reference (P) andthe survey tool frame (T)—determined in part using land surveyingprocedures (orientation of x and y tool axes). The orientation of thetool about its longitudinal (z) axis is more difficult to control,particularly if the oil platform on which the initialization process istaking place is moving. To overcome this concern, the following methodcan be used.

The high side of the tool 530 can be established to a relatively highdegree of accuracy using the tool accelerometer measurements providedthat the tool 530 is substantially stationary. Thus, one example methodincludes determining the tool highside on land (as part of the toolcalibration process) and affixing (e.g., clamping) a sleeve 560 to thetool casing with reference structures, e.g., clearly defined protrusions562, in a known position(s) with respect to the x and y axes of theinstrument assembly within the tool—as schematically illustrated in FIG.19. This sleeve assembly 560 then remains attached to the tool 530 whileit is shipped to the offshore platform. The assembly 570 in which thesurvey tool 530 is to be mounted (e.g., clamped) on the platform can bedesigned to allow the tool protrusions 562 to key into a correspondingmechanism on the platform to lock the tool 530 in a predeterminedorientation about its z-axis, as illustrated in FIG. 20. Thus, incertain embodiments, the wellbore survey tool 530 is mounted at apredetermined orientation with respect to the directional referencesystem 540 using corresponding keying structures affixed to a mount thatis located at the first position.

Other methods of achieving the same or similar result involve thesubstantially rigid attachment of a cross-over piece to one end of thesurvey tool, to which a key way can be machined; either a protrusion oran indentation in the cross-over, for example.

The attitude of the survey tool with respect to the geographic frame(C_(G) ^(T)) may then be calculated using the following matrix equation:

C _(G) ^(T) =C _(G) ^(R) C _(R) ^(P) C _(P) ^(T)

where C_(R) ^(P) is equal to the transpose of the matrix C_(P) ^(R).

One object of this particular scheme is to initialize the survey tool530 while positioned above the well in the derrick, although the methodis generally applicable for any tool orientation; vertical to horizontalon the rig. The tool 530 may be fully made up prior to the start of theinitialization process, ready to be inserted into the wellbore, andclamped in position at its two ends (e.g., at the ends of tool sectioncontaining the instrument assembly). Land surveying techniques may beused to establish the position of the end supports, thus defining thetool orientation about its lateral (x and y) axes with respect to theplatform reference axis set. The sleeve assembly 560 attached to thecasing of the tool prior to shipment offshore and the clamping assembly570 on the rig can be used to define the tool orientation about thez-axis.

FIG. 21 shows the example locations of the directional reference system540 and the survey tool 530 in which the initialization process is totake place. The survey tool 530 can be held by tool initializationsupport 580 (including clamping assembly 570) of the derrick 590 andspaced away from the directional reference system 540.

2. Methods Involving the Use of an Additional Inertial Reference System

Certain alternative methods for initializing a gyro survey tool 530 aredescribed next. According to some embodiments, these alternative methodsare not reliant on and/or may not involve optical measurements andlasers. As described more fully below, values received from an inertialreference system can be used to determine the orientation of thewellbore survey tool 530 at the first position.

FIG. 22 shows a reference platform containing the directional referencesystem 540 (e.g., GPS system) comprising satellite antennae 542 (two ormore) and a survey tool 530 located at a location remote from thedirectional reference system 540. The method shown here involves theapplication of an inertial attitude and heading reference system (AHRS)unit 600 to store the azimuth reference set up using the directionalreference system 540. This result can be achieved by initially mountingthe AHRS unit 600 on the reference platform of the directional referencesystem 540. Having transferred the satellite reference to the AHRS unit600, it can be detached from the platform and physically moved orcarried to the entrance to the well where it can be affixed (e.g.,clamped) to a platform to which the tool 530 is also attached. Assumingthat the AHRS unit 600 and the tool 530 are accurately aligned relativeto one another, or their relative orientation is known to sufficientaccuracy, the azimuth defined by the AHRS unit 600 may be transferred tothe survey tool 530.

For example, the reference azimuth (A_(R)) can be determined using thedirectional reference system 540 and can be transferred to the AHRS unit600. While the AHRS unit 600 is carried to the wellbore entrance, theAHRS unit 600 maintains the attitude reference throughout. The AHRS unit600 can then be attached to mounting blocks to which the survey tool 530is also attached, and the attitude reference from the AHRS unit 600 canthen be transferred to the survey tool 530. The survey tool 530 can thenbe switched to continuous survey mode and rotated to vertical above thewellbore entrance. Thus, in certain embodiments, before the orientationof the wellbore survey tool 530 is determined, the inertial referencesystem (e.g., AHRS unit 600) is moved from a first mounting position inwhich the inertial reference system is mounted at a predeterminedorientation with respect to the directional reference system 540 to asecond mounting position in which the inertial reference system ismounted at a predetermined orientation with respect to the wellboresurvey tool 530.

The accuracy of the method involving the physical transfer of the AHRSunit 600 to the tool location can depend to some degree on the accuracywith which the AHRS unit 600 can be aligned mechanically in itsrespective mounting locations; firstly to the satellite antennaestructure of the directional reference system 540 and subsequently tothe survey tool 530. This alignment can be more challenging with thetool vertical, since the length of the baseline which controls theaccuracy of this alignment may only be a few centimeters (the diameterof the tool) compared to meters (the length of the tool) in the casewhere the tool 530 is horizontal. However, the method described earlierof setting up a key way during tool assembly to define the orientationof the tool when affixed or clamped in place on the rig may be used(ref. FIGS. 19 and 20).

In certain cases, a significant advantage of this method, compared tothe optical sighting methods described above, is a reduced dependency onthe degree of rigidity of the rig structure. For example, the mountingarrangement over the relatively short distances between the AHRS unit600 and the satellite antennae structure of the directional referencesystem 540, and between the AHRS unit 600 and the tool 530, are relevantto such a method.

A further option, which according to certain embodiments does notinvolve the physical transport of the AHRS unit 600 between thereference site of the directional reference system 540 and the locationof the tool 530, is shown in FIG. 23. In this case, angular ratemeasurements generated by the AHRS unit 600 and the gyroscopes in thesurvey tool 530 are compared and used to determine the relativeorientation of the tool 530 and the AHRS unit 600 in a process referredto as inertial measurement matching. The time taken to perform thisoperation, and the accuracy to which it can be accomplished, can be afunction of the motion of the rig or drilling platform on which thesystem is located. Given knowledge of the reference orientation(generated using the satellite system) to which the AHRS unit 600 isphysically aligned and the relative orientation to the tool 530, asdescribed above, the orientation of the tool 530 with respect to truenorth can be calculated. This information is then used to initialize thesurvey tool 530 before engaging continuous survey mode.

For example, the reference azimuth (A_(R)) can be determined using thedirectional reference system 540 and can be transferred to the AHRS unit600. A comparison of the angular rate measured by the AHRS unit 600 andmeasured by the survey tool 530 can be performed by the processor 610,which can then determine the relative attitude (ΔA) between the AHRSunit 600 and the tool 530. The tool azimuth can then be expressed asA_(T)=A_(R)−ΔA. The tool 530 can then be switched to continuous surveymode and rotated to vertical above the wellbore.

Both methods involving the use of the AHRS unit 600 may be implementedwith the survey tool 530 either vertical or horizontal, or anywhere inbetween.

In an alternative configuration, when the tool 530 is vertical orsubstantially vertical, a large spinning wheel (spinning vertically) isset in a full gravity weighted gimbal system. The gimbal system may havea window on the top of the box to see the gyro tool face angle, forexample. One example usage of such a configuration is to attach thedirectional reference system 540 (e.g., GPS unit or fixture) and spin upin the reference position and then detach and move to the rig floorwhere it gets attached to the tool 530 (e.g., to a tool referenceplate). Then the tool 530 can be turned in the gyro tool face planeuntil the AHRS unit 600 is back at its reference position, and thesurvey tool initialisation can be performed.

Although certain preferred embodiments and examples are discussed above,it is understood that the inventive subject matter extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. It is intended that the scope of the inventions disclosedherein should not be limited by the particular disclosed embodiments.Thus, for example, in any method or process disclosed herein, the actsor operations making up the method/process may be performed in anysuitable sequence and are not necessarily limited to any particulardisclosed sequence. Various aspects and advantages of the embodimentshave been described where appropriate. It is to be understood that notnecessarily all such aspects or advantages may be achieved in accordancewith any particular embodiment. Thus, for example, it should berecognized that the various embodiments may be carried out in a mannerthat achieves or optimizes one advantage or group of advantages astaught herein without necessarily achieving other aspects or advantagesas may be taught or suggested herein.

1. A method for determining an orientation of a wellbore survey tool ata first position with respect to a reference direction, the methodcomprising: receiving at least one first signal indicative of anorientation of a directional reference system with respect to thereference direction, the directional reference system positioned at asecond position spaced from the first position; receiving at least onesecond signal indicative of a relative orientation of the wellboresurvey tool with respect to the directional reference system; anddetermining the orientation of the wellbore survey tool at the firstposition in response at least in part to the at least one first signaland the at least one second signal.
 2. The method of claim 1, whereinthe first position is spaced a first distance from a wellbore entrance,and the second position is spaced a second distance from the wellboreentrance.
 3. The method of claim 2, wherein a first horizontal componentof the first distance is less than about 10 feet.
 4. The method of claim2, wherein a second horizontal component of the second distance isgreater than a first horizontal component of the first distance by atleast about 30 feet.
 5. The method of claim 2, wherein a first verticalcomponent of the first distance is less than about 20 feet.
 6. Themethod of claim 1, wherein determining the orientation of the wellboresurvey tool at the first position comprises: using one or more computerprocessors to receive the at least one first signal and the at least onesecond signal; and using the one or more computer processors tocalculate the orientation of the wellbore survey tool at the firstposition.
 7. The method of claim 6, further comprising using one or moreoptical sighting procedures to generate information of the at least onesecond signal indicative of the relative orientation of the wellboresurvey tool with respect to the directional reference system.
 8. Themethod of claim 7, wherein the one or more optical sighting procedurescomprises: transmitting light along a first line extending between thedirectional reference system and a first reflecting surface mounted at apredetermined orientation with respect to the wellbore survey tool; andreflecting the light transmitted along the first line by the firstreflecting surface.
 9. The method of claim 8, wherein the wellboresurvey tool at the first position is mounted substantially verticallywith respect to the wellbore entrance.
 10. The method of claim 9,further comprising moving the first reflecting surface to change thedirection the light reflected by the first reflecting surface, whereinthe first reflecting surface and the wellbore survey tool aremechanically coupled to one another and maintain their relationship withone another while being moved.
 11. The method of claim 8, wherein theone or more optical sighting procedures further comprises: transmittingthe light reflected by the first reflecting surface along a second lineextending between the first reflecting surface and a second reflectingsurface; and using the second reflecting surface to reflect the lighttransmitted along the second line.
 12. The method of claim 11, whereinthe light reflected by the second reflecting surface propagates along athird line extending between the second reflecting surface and thedirectional reference system, and wherein the first line, the secondline, and the third line form a triangle.
 13. The method of claim 1,further comprising mounting the wellbore survey tool at a predeterminedorientation with respect to the directional reference system by engagingkeying features affixed to the wellbore survey tool with correspondingkeying structures affixed to a mount that is located at the firstposition.
 14. The method of claim 1, further comprising: determining thetool highside angle while the wellbore survey tool is substantiallyhorizontal; moving the wellbore survey tool to be substantiallyvertical, wherein the determining the orientation of the wellbore surveytool at the first position comprises calculating a gyro tool face angleat least in part based on the determined tool highside angle.
 15. Themethod of claim 14, the tool highside angle is determined usingaccelerometer measurements from the wellbore survey tool.
 16. The methodof claim 15, wherein the tool highside angle is determined while thewellbore survey tool is mounted in a v-shaped channel that has beenaligned with the local horizontal using a level sensor.
 17. The methodof claim 1, further comprising using values received from an inertialreference system to determine the orientation of the wellbore surveytool at the first position.
 18. The method of claim 17, wherein, beforethe determining the orientation of the wellbore survey tool at the firstposition is performed, the inertial reference system is moved from afirst mounting position in which the inertial reference system ismounted at a predetermined orientation with respect to the directionalreference system to a second mounting position in which the inertialreference system is mounted at a predetermined orientation with respectto the wellbore survey tool.
 19. The method of claim 17, wherein thedetermining the orientation of the wellbore survey tool at the firstposition further comprises comparing angular rate measurements by thewellbore survey tool and the inertial reference system.
 20. The methodof claim 17, wherein the inertial reference system comprises an attitudeand heading reference system (AHRS).
 21. A system for determining anorientation of a wellbore survey tool, the system comprising: one ormore computer processors; one or more inputs configured to receive dataindicative of an orientation of a directional reference system withrespect to a reference direction and data indicative of a relativeorientation of the wellbore survey tool with respect to the directionalreference system, wherein the direction reference system is positionedat a first position relative to a wellbore entrance and a wellboresurvey tool is mounted at a second position relative to the wellboreentrance spaced away from the first position; and a wellboreinitialization module executing in the one or more computer processorsand configured to, in response at least in part to the received data,calculate an orientation of the survey tool.
 22. A system for use indetermining an orientation of a wellbore survey tool, the systemcomprising: at least one directional reference system configured toprovide data indicative of an orientation of the at least onedirectional reference system with respect to a reference direction; andan optical component mounted at a predetermined orientation with respectto the directional reference system and configured to transmit lightalong a line extending between the directional reference system and afirst reflecting surface mounted at a predetermined orientation withrespect to the wellbore survey tool.
 23. The system of claim 22, furthercomprising a second reflecting surface positioned such that lightreflected by the first reflecting surface propagates along a second lineextending between the wellbore survey tool and the second reflectingsurface and is reflected by the second reflecting surface.
 24. Thesystem of claim 23, wherein the light reflected by the second reflectingsurface extends along a third line between the second reflecting surfaceand the directional reference system, wherein the first line, the secondline, and the third line form a triangle.
 25. The system of claim 24,further comprising an actuator configured to move the second reflectingsurface along the third line.
 26. The system of claim 22, furthercomprising an autocollimator.